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The Blackthorn Reach Mass Psychogenic Illness Disaster, commonly known as The Observer Syndrome, was a fictional anomalous public health event that occurred in the town of Blackthorn Reach, Wyoming, in 2026. The incident was initially classified as a mass psychogenic illness but was later reclassified by investigators as Perceptually Transmitted Psychogenic Syndrome (PTPS) after numerous cases demonstrated progressive neurological deterioration inconsistent with known psychiatric disorders.

Joseph Brown (born 1970) is the only documented survivor of the Blackthorn Reach Mass Psychogenic Illness Disaster; his apparent lack of symptoms has made him a subject of interest to researchers.

Blackthorn Reach High School was the sole secondary school serving Blackthorn Reach. The campus and teams (the Blackthorn Ravens) were central to the town before the 2029 evacuation.

Blackthorn Reach Community Hospital was a 42-bed critical access hospital in Blackthorn Reach, Wyoming, notable for its role during the 2029 outbreak that later became associated with The Observer Syndrome.

Blackthorn Reach is a ghost town in northeastern Wyoming. Formerly home to more than 1,600 residents, the town was evacuated and abandoned following the Blackthorn Reach Mass Psychogenic Illness Disaster in 2029.

Catalysis (, kə-TAL-iss-iss) is the increase in rate of a chemical reaction due to an added substance known as a catalyst ( KAT-əl-ist). Catalysts are not consumed by the reaction and remain unchanged after the reaction. If the reaction is rapid and the catalyst is recycled quickly, a very small amount of catalyst often suffices; mixing, surface area, and temperature are important factors in reaction rate. Catalysts generally react with one or more reactants to form intermediates that subsequently give the final reaction product, in the process of regenerating the catalyst. The rate increase occurs because the catalyst allows the reaction to occur by an alternative mechanism which may be much faster than the noncatalyzed mechanism. However the noncatalyzed mechanism does remain possible, so that the total rate (catalyzed plus noncatalyzed) can only increase in the presence of the catalyst and never decrease. Catalysis may be classified as either homogeneous, whose components are dispersed in the same phase (usually gaseous or liquid) as the reactant, or heterogeneous, whose components are not in the same phase. Enzymes and other biocatalysts are often considered as a third category. Catalysis is nearly ubiquitous in the chemical industry. Estimates are that 90% of all commercially produced chemical products involve catalysts at some stage in the process of their manufacture. The term "catalyst" is derived from Greek καταλύειν, kataluein, meaning "loosen" or "untie". The concept of catalysis was invented by chemist Elizabeth Fulhame, based on her novel work in oxidation-reduction experiments.

In chemistry, there are three definitions in common use of the word "base": Arrhenius bases, Brønsted bases, and Lewis bases. All definitions agree that bases are substances that react with acids, as originally proposed by G.-F. Rouelle in the mid-18th century. In 1884, Svante Arrhenius proposed that a base is a substance which dissociates in aqueous solution to form hydroxide ions OH−. These ions can react with hydrogen ions (H+ according to Arrhenius) from the dissociation of acids to form water in an acid–base reaction. A base was therefore a metal hydroxide such as NaOH or Ca(OH)2. Such aqueous hydroxide solutions were also described by certain characteristic properties. They are slippery to the touch, can taste bitter, and change the color of pH indicators (e.g., turn red litmus paper blue). In water, by altering the autoionization equilibrium, bases yield solutions in which the hydrogen ion activity is lower than it is in pure water, i.e., the water has a pH higher than 7.0 at standard conditions. A soluble base is called an alkali if it contains and releases OH− ions quantitatively. Metal oxides, hydroxides, and especially alkoxides are basic, and conjugate bases of weak acids are weak bases. Bases and acids are seen as chemical opposites because the effect of an acid is to increase the hydronium (H3O+) concentration in water, whereas bases reduce this concentration. A reaction between aqueous solutions of an acid and a base is called neutralization, producing a solution of water and a salt in which the salt separates into its component ions. If the aqueous solution is saturated with a given salt solute, any additional such salt precipitates out of the solution. In the more general Brønsted–Lowry acid–base theory (1923), a base is a substance that can accept hydrogen cations (H+)—otherwise known as protons. This does include aqueous hydroxides since OH− does react with H+ to form water, so that Arrhenius bases are a subset of Brønsted bases. However, there are also other Brønsted bases which accept protons, such as aqueous solutions of ammonia (NH3) or its organic derivatives (amines). These bases do not contain a hydroxide ion but nevertheless react with water, resulting in an increase in the concentration of hydroxide ion. Also, some non-aqueous solvents contain Brønsted bases which react with solvated protons. For example, in liquid ammonia, NH2− is the basic ion species which accepts protons from NH4+, the acidic species in this solvent. G. N. Lewis realized that water, ammonia, and other bases can form a bond with a proton due to the unshared pair of electrons that the bases possess. In the Lewis theory, a base is an electron pair donor which can share a pair of electrons with an electron acceptor which is described as a Lewis acid. The Lewis theory is more general than the Brønsted model because the Lewis acid is not necessarily a proton, but can be another molecule (or ion) with a vacant low-lying orbital which can accept a pair of electrons. One notable example is boron trifluoride (BF3). Some other definitions of both bases and acids have been proposed in the past, but are not commonly used today.

Oxygen is a chemical element; it has the symbol O and its atomic number is 8. It is a member of the chalcogen group in the periodic table. It is highly reactive, a nonmetal, and a potent oxidizing agent that readily forms oxides with most elements as well as with other compounds. Oxygen is the most abundant element in Earth's crust, making up almost half of the Earth's crust in the form of various oxides such as water, carbon dioxide, iron oxides, and silicates. It is also the third-most abundant element in the universe after hydrogen and helium. At standard temperature and pressure, two oxygen atoms will bind covalently to form dioxygen, a colorless and odorless diatomic gas with the chemical formula O2. Dioxygen gas currently constitutes approximately 20.95% molar fraction of the Earth's atmosphere, though this has changed considerably over long periods of time in Earth's history. The much rarer allotrope of oxygen, ozone (O3), strongly absorbs the UVB and UVC wavelengths and forms a protective ozone layer at the lower stratosphere, which shields the biosphere from ionizing ultraviolet radiation. However, ozone present at the surface is a corrosive byproduct of smog and thus an air pollutant. All eukaryotic organisms, including plants, animals, fungi, algae, and most protists, need oxygen for cellular respiration, a process that extracts chemical energy by the reaction of oxygen with organic molecules derived from food and releases carbon dioxide as a waste product. Many major classes of organic molecules in living organisms contain oxygen atoms, such as proteins, nucleic acids, carbohydrates, and fats, as do the major constituent inorganic compounds of animal shells, teeth, and bone. Most of the mass of living organisms is oxygen as a component of water, the major constituent of lifeforms. Oxygen in Earth's atmosphere is produced by biotic photosynthesis, in which photon energy in sunlight is captured by chlorophyll to split water molecules and then react with carbon dioxide to produce carbohydrates, with oxygen released as a byproduct. Oxygen is too chemically reactive to remain a free element in air without being continuously replenished by the photosynthetic activities of autotrophs such as cyanobacteria, chloroplast-bearing algae, and land plants. Oxygen was isolated by Michael Sendivogius before 1604, but it is commonly believed that the element was discovered independently by Carl Wilhelm Scheele, in Uppsala, in 1773 or earlier, and Joseph Priestley in Wiltshire, in 1774. Priority is often given for Priestley because his work was published first. Priestley, however, called oxygen "dephlogisticated air", and did not recognize it as a chemical element. In 1777, Antoine Lavoisier first recognized oxygen as a chemical element and correctly characterized the role it plays in combustion. Common industrial uses of oxygen include production of steel, plastics and textiles, brazing, welding and cutting of steels and other metals, rocket propellant, oxygen therapy, and life-support systems in aircraft, submarines, spaceflight, and diving.

Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combustion does not always result in fire, because a flame is only visible when substances undergoing combustion vaporize, but when it does, a flame is a characteristic indicator of the reaction. While activation energy must be supplied to initiate combustion (e.g., using a lit match to light a fire), the heat from a flame may provide enough energy to make the reaction self-sustaining. The study of combustion is known as combustion science. Combustion is often a complicated sequence of elementary radical reactions. Solid fuels, such as wood and coal, first undergo endothermic pyrolysis to produce gaseous fuels whose combustion then supplies the heat required to produce more of them. Combustion is often hot enough that incandescent light in the form of either glowing or a flame is produced. A simple example can be seen in the combustion of hydrogen and oxygen into water vapor, a reaction which is commonly used to fuel rocket engines. This reaction releases 242 kJ/mol of heat and reduces the enthalpy accordingly (at constant temperature and pressure): 2 H 2 ( g ) + O 2 ( g ) → 2 H 2 O ↑ {\displaystyle {\ce {2H_{2}(g){+}O_{2}(g)\rightarrow 2H_{2}O\uparrow }}} Uncatalyzed combustion in air requires relatively high temperatures. Complete combustion is stoichiometric concerning the fuel, where there is no remaining fuel, and ideally, no residual oxidant. Thermodynamically, the chemical equilibrium of combustion in air is overwhelmingly on the side of the products. However, complete combustion is almost impossible to achieve, since the chemical equilibrium is not necessarily reached, or may contain unburnt products such as carbon monoxide, hydrogen and even carbon (soot or ash). Thus, the produced smoke is usually toxic and contains unburned or partially oxidized products. Any combustion at high temperatures in atmospheric air, which is 78 percent nitrogen, will also create small amounts of several nitrogen oxides, commonly referred to as NOx, since the combustion of nitrogen is thermodynamically favored at high, but not low temperatures. Since burning is rarely clean, fuel gas cleaning or catalytic converters may be required by law. Fires occur naturally, ignited by lightning strikes or by volcanic products. Combustion (fire) was the first controlled chemical reaction discovered by humans, in the form of campfires and bonfires, and continues to be the main method to produce energy for humanity. Usually, the fuel is hydrogen, hydrocarbons (a compound with a mixture of hydrogen and carbon), or more complicated mixtures such as wood that contain partially oxidized hydrocarbons. The thermal energy produced from the combustion of either fossil fuels such as coal or oil, or from renewable fuels such as firewood, is harvested for diverse uses such as cooking, production of electricity or industrial or domestic heating. Combustion is also currently the only reaction used to power rockets. Combustion is also used to destroy (incinerate) waste, both nonhazardous and hazardous. Oxidants for combustion have high oxidation potential and include atmospheric or pure oxygen, chlorine, fluorine, chlorine trifluoride, nitrous oxide and nitric acid. For instance, hydrogen burns in chlorine to form hydrogen chloride with the liberation of heat and light characteristic of combustion. Although usually not catalyzed, combustion can be catalyzed by platinum or vanadium, as in the contact process.

In chemistry, a solution is defined by IUPAC as "A liquid or solid phase containing more than one substance, when for convenience one (or more) substance, which is called the solvent, is treated differently from the other substances, which are called solutes. When, as is often but not necessarily the case, the sum of the mole fractions of solutes is small compared with unity, the solution is called a dilute solution." One parameter of a solution is the concentration, which is a measure of the amount of solute in a given amount of solution or solvent. The term "aqueous solution" is used when one of the solvents is water.

Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, and reactions of organic compounds and organic materials (i.e. matter in its various forms that contain carbon atoms). It involves studying the structure of organic material to determine the structural formula, analyzing physical and chemical properties, and evaluating chemical reactivity to understand the behavior of organic compounds. The study of organic reactions includes the chemical synthesis of natural products, drugs, and polymers, and study of individual organic molecules in the laboratory and via theoretical (in silico) study. The range of chemicals studied in organic chemistry includes hydrocarbons (compounds containing only carbon and hydrogen) as well as compounds based on carbon, but also containing other elements, especially oxygen, nitrogen, sulfur, phosphorus (included in many biochemicals) and the halogens. Organometallic chemistry is the study of compounds containing carbon–metal bonds. Organic compounds form the basis of all known life and constitute the majority of known chemicals. The bonding patterns of carbon, with its valence of four—formal single, double, and triple bonds, plus structures with delocalized electrons—make the array of organic compounds structurally diverse, and their range of applications enormous. They form the basis of, or are constituents of, many commercial products including pharmaceuticals; petrochemicals and agrichemicals, and products made from them including lubricants, solvents; plastics; fuels and explosives. The study of organic chemistry overlaps organometallic chemistry and biochemistry, but also with medicinal chemistry, polymer chemistry, and materials science.

Biochemistry, or biological chemistry (distinct from chemical biology), is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology, and metabolism. Over the last decades of the 20th century, biochemistry has become successful at explaining living processes through these three disciplines. Almost all areas of the life sciences are being uncovered and developed through biochemical methodology and research. Biochemistry focuses on understanding the chemical basis that allows biological molecules to give rise to the processes that occur within living cells and between cells, in turn relating greatly to the understanding of tissues and organs as well as organism structure and function. Biochemistry is closely related to molecular biology, the study of the molecular mechanisms of biological phenomena. Much of biochemistry deals with the structures, functions, and interactions of biological macromolecules such as proteins, nucleic acids, carbohydrates, and lipids. They provide the structure of cells and perform many of the functions associated with life. The chemistry of the cell also depends upon the reactions of small molecules and ions. These can be inorganic (for example, water and metal ions) or organic (for example, the amino acids, which are used to synthesize proteins). The mechanisms used by cells to harness energy from their environment via chemical reactions are known as metabolism. The findings of biochemistry are applied primarily in medicine, nutrition, and agriculture. In medicine, biochemists investigate the causes and cures of diseases. Nutrition studies how to maintain health and wellness and also the effects of nutritional deficiencies. In agriculture, biochemists investigate soil and fertilizers with the goal of improving crop cultivation, crop storage, and pest control. In recent decades, biochemical principles and methods have been combined with problem-solving approaches from engineering to manipulate living systems in order to produce useful tools for research, industrial processes, and diagnosis and control of disease—the discipline of biotechnology.

Carbon (from Latin carbo 'coal') is a chemical element; it has symbol C and atomic number 6. It is nonmetallic and tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 electrons. It belongs to group 14 of the periodic table. Carbon makes up about 0.025 percent of Earth's crust. Three isotopes occur naturally, 12C and 13C being stable, while 14C is a radionuclide, decaying with a half-life of 5,700 years. Carbon is one of the few elements known since antiquity. Carbon is the 15th most abundant element in the Earth's crust, and the fourth most abundant element in the universe by mass after hydrogen, helium, and oxygen. Carbon's abundance, its unique diversity of organic compounds, and its unusual ability to form polymers at the temperatures commonly encountered on Earth, enables it to serve as the common element of all known life. It is the second most abundant element in the human body by mass (about 18.5%) after oxygen. The atoms of carbon can bond together in diverse ways, resulting in various allotropes of carbon. Well-known allotropes include graphite, diamond, amorphous carbon, and fullerenes. The physical properties of carbon vary widely with the allotropic form. For example, graphite is opaque and black, while diamond is highly transparent. Graphite is soft enough to form a streak on paper (hence its name, from the Greek verb "γράφειν" which means "to write"), while diamond is the hardest naturally occurring material known. Graphite is a good electrical conductor while diamond has a low electrical conductivity. Under normal conditions, diamond, carbon nanotubes, and graphene have the highest thermal conductivities of all known materials. All carbon allotropes are solids under normal conditions, with graphite being the most thermodynamically stable form at standard temperature and pressure. They are chemically resistant and require high temperature to react even with oxygen. The most common oxidation state of carbon in inorganic compounds is +4, while +2 is found in carbon monoxide and transition metal carbonyl complexes. The largest sources of inorganic carbon are limestones, dolomites and carbon dioxide, but significant quantities occur in organic deposits of coal, peat, oil, and methane clathrates. Carbon forms a vast number of compounds, with about two hundred million having been described and indexed; and yet that number is still only a fraction of the number of theoretically possible compounds under standard conditions.

An acid is a molecule or ion capable of either donating a proton (i.e. hydrogen cation, H+), known as a Brønsted–Lowry acid, or forming a covalent bond with an electron pair, known as a Lewis acid. The first category of acids are the proton donors, or Brønsted–Lowry acids. In the special case of aqueous solutions, proton donors form the hydronium ion H3O+ and are known as Arrhenius acids. Brønsted and Lowry generalized the Arrhenius theory to include non-aqueous solvents. A Brønsted–Lowry or Arrhenius acid usually contains a hydrogen atom bonded to a chemical structure that is still energetically favorable after loss of H+. Aqueous Arrhenius acids have characteristic properties that provide a practical description of an acid. Acids form aqueous solutions with a sour taste, can turn blue litmus red, and react with bases and certain metals (like calcium) to form salts. The word acid is derived from the Latin acidus, meaning 'sour'. An aqueous solution of an acid has a pH less than 7 and is colloquially also referred to as "acid" (as in "dissolved in acid"), while the strict definition refers only to the solute. A lower pH means a higher acidity, and thus a higher concentration of hydrogen cations in the solution. Chemicals or substances having the property of an acid are said to be acidic. Common aqueous acids include hydrochloric acid (a solution of hydrogen chloride that is found in gastric acid in the stomach and activates digestive enzymes), acetic acid (vinegar is a dilute aqueous solution of this liquid), sulfuric acid (used in car batteries), and citric acid (found in citrus fruits). As these examples show, acids (in the colloquial sense) can be solutions or pure substances, and can be derived from acids (in the strict sense) that are solids, liquids, or gases. Strong acids and some concentrated weak acids are corrosive, but there are exceptions such as carboranes and boric acid. The second category of acids are Lewis acids, which form a covalent bond with an electron pair. An example is boron trifluoride (BF3), whose boron atom has a vacant orbital that can form a covalent bond by sharing a lone pair of electrons on an atom in a base, for example the nitrogen atom in ammonia (NH3). Lewis considered this as a generalization of the Brønsted definition, so that an acid is a chemical species that accepts electron pairs either directly or by releasing protons (H+) into the solution, which then accept electron pairs. Hydrogen chloride, acetic acid, and most other Brønsted–Lowry acids cannot form a covalent bond with an electron pair, however, and are therefore not Lewis acids. Conversely, many Lewis acids are not Arrhenius or Brønsted–Lowry acids. In modern terminology, an acid is implicitly a Brønsted acid and not a Lewis acid, since chemists almost always refer to a Lewis acid explicitly as such.

Electrochemistry is the branch of physical chemistry concerned with the relationship between electrical potential difference and identifiable chemical change. These reactions involve electrons moving via an electronically conducting phase (typically an external electric circuit, but not necessarily, as in electroless plating) between electrodes separated by an ionically conducting and electronically insulating electrolyte (or ionic species in a solution). When a chemical reaction is driven by an electrical potential difference, as in electrolysis, or if a potential difference results from a chemical reaction as in an electric battery or fuel cell, it is called an electrochemical reaction. In electrochemical reactions, unlike in other chemical reactions, electrons are not transferred directly between atoms, ions, or molecules, but via the aforementioned electric circuit. This phenomenon is what distinguishes an electrochemical reaction from a conventional chemical reaction.

A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. When chemical reactions occur, the atoms are rearranged and the reaction is accompanied by an energy change as new products are generated. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and breaking of chemical bonds between atoms, with no change to the nuclei (no change to the elements present), and can often be described by a chemical equation. Nuclear chemistry is a sub-discipline of chemistry that involves the chemical reactions of unstable and radioactive elements where both electronic and nuclear changes can occur. The substance (or substances) initially involved in a chemical reaction are called reactants or reagents. Chemical reactions are usually characterized by a chemical change, and they yield one or more products, which usually have properties different from the reactants. Reactions often consist of a sequence of individual sub-steps, the so-called elementary reactions, and the information on the precise course of action is part of the reaction mechanism. Chemical reactions are described with chemical equations, which symbolically present the starting materials, end products, and sometimes intermediate products and reaction conditions. Chemical reactions happen at a characteristic reaction rate at a given temperature and chemical concentration. Some reactions produce heat and are called exothermic reactions, while others may require heat to enable the reaction to occur, which are called endothermic reactions. Typically, reaction rates increase with increasing temperature because there is more thermal energy available to reach the activation energy necessary for breaking bonds between atoms. A reaction may be classified as redox in which oxidation and reduction occur or non-redox in which there is no oxidation and reduction occurring. Most simple redox reactions may be classified as a combination, decomposition, or single displacement reaction. Different chemical reactions are used during chemical synthesis in order to obtain the desired product. In biochemistry, a consecutive series of chemical reactions (where the product of one reaction is the reactant of the next reaction) form metabolic pathways. These reactions are often catalyzed by protein enzymes. Enzymes increase the rates of biochemical reactions, so that metabolic syntheses and decompositions impossible under ordinary conditions can occur at the temperature and concentrations present within a cell. The general concept of a chemical reaction has been extended to reactions between entities smaller than atoms, including nuclear reactions, radioactive decays and reactions between elementary particles, as described by quantum field theory.

An enzyme is a biological macromolecule, usually a protein, that acts as a biological catalyst, accelerating chemical reactions without being consumed in the process. The molecules on which enzymes act are called substrates, which are converted into products. Nearly all metabolic processes within a cell depend on enzyme catalysis to occur at biologically relevant rates. A metabolic pathway is typically composed of a series of enzyme-catalyzed steps. The study of enzymes is known as enzymology, and a related field focuses on pseudoenzymes—proteins that have lost catalytic activity but may retain regulatory or scaffolding functions, often indicated by alterations in their amino acid sequences or unusual 'pseudocatalytic' behavior. Enzymes are known to catalyze over 5,000 types of biochemical reactions. Other biological catalysts include catalytic RNA molecules, or ribozymes, which are sometimes classified as enzymes despite being composed of RNA rather than protein. More recently, biomolecular condensates have been recognized as a third category of biocatalysts, capable of catalyzing reactions by creating interfaces and gradients—such as ionic gradients—that drive biochemical processes, even when their component proteins are not intrinsically catalytic. Enzymes increase the reaction rate by lowering a reaction's activation energy, often by factors of millions. A striking example is orotidine 5′-phosphate decarboxylase, which accelerates a reaction that would otherwise take millions of years to occur in milliseconds. Like all catalysts, enzymes do not affect the overall equilibrium of a reaction and are regenerated at the end of each cycle. What distinguishes them is their high specificity, determined by their unique three-dimensional structure, and their sensitivity to factors such as temperature and pH. Enzyme activity can be enhanced by activators or diminished by inhibitors, many of which serve as drugs or poisons. Outside optimal conditions, enzymes may lose their structure through denaturation, leading to loss of function. Enzymes have widespread practical applications. In industry, they are used to catalyze the production of antibiotics and other complex molecules. In everyday life, enzymes in biological washing powders break down protein, starch, and fat stains, enhancing cleaning performance. Papain and other proteolytic enzymes are used in meat tenderizers to hydrolyze proteins, improving texture and digestibility. Their specificity and efficiency make enzymes indispensable in both biological systems and commercial processes.

Nanotechnology is the manipulation of matter with at least one dimension sized from 1 to 100 nanometers (nm). At this scale, commonly known as the nanoscale, surface area and quantum mechanical effects become important in describing properties of matter. This definition of nanotechnology includes all types of research and technologies that deal with these special properties. It is common to see the plural form "nanotechnologies" as well as "nanoscale technologies" to refer to research and applications whose common trait is scale. An earlier understanding of nanotechnology referred to the particular technological goal of precisely manipulating atoms and molecules for fabricating macroscale products, now referred to as molecular nanotechnology. Nanotechnology defined by scale includes fields of science such as surface science, organic chemistry, molecular biology, semiconductor physics, energy storage, engineering, microfabrication, and molecular engineering. The associated research and applications range from extensions of conventional device physics to molecular self-assembly, from developing new materials with dimensions on the nanoscale to direct control of matter on the atomic scale. A near-synonym of "nanotechnology" is nanoscience (e.g., nano-optics); however, the latter emphasizes the pure science aspects of the field. Nanotechnology may be able to create new materials and devices with diverse applications, such as in nanomedicine, nanoelectronics, agricultural sectors, biomaterials energy production, and consumer products. However, nanotechnology raises issues, including concerns about the toxicity and environmental impact of nanomaterials, and their potential effects on global economics, as well as various doomsday scenarios. These concerns have led to a debate among advocacy groups and governments on whether special regulation of nanotechnology is warranted.

The Nobel Prize in Chemistry is awarded annually by the Royal Swedish Academy of Sciences to scientists in the various fields of chemistry. It is one of the five Nobel Prizes established by the will of Alfred Nobel in 1895, awarded for outstanding contributions in chemistry, physics, literature, peace, and physiology or medicine. This award is administered by the Nobel Foundation and awarded by the Royal Swedish Academy of Sciences on proposal of the Nobel Committee for Chemistry, which consists of five members elected by the academy. The award is presented in Stockholm at an annual ceremony on December 10, the anniversary of Nobel's death. The first Nobel Prize in Chemistry was awarded in 1901 to Jacobus Henricus van 't Hoff, of the Netherlands, "for his discovery of the laws of chemical dynamics and osmotic pressure in solutions". From 1901 to 2025, the award has been bestowed on a total of 198 individuals. Frederick Sanger and K. Barry Sharpless both won twice. The 2025 Nobel Prize in Chemistry was awarded to Susumu Kitagawa, Richard Robson and Omar M. Yaghi "for the development of metal–organic frameworks". As of 2022, eight women had won the prize: Marie Curie (1911), her daughter Irène Joliot-Curie (1935), Dorothy Hodgkin (1964), Ada Yonath (2009), Frances Arnold (2018), Emmanuelle Charpentier and Jennifer Doudna (2020), and Carolyn R. Bertozzi (2022). Marie Curie also won the Nobel Prize in Physics in 1903, making her the only person to win in two different sciences.

Deoxyribonucleic acid (; DNA) is a polymer composed of two polynucleotide chains that coil around each other to form a double helix. The polymer carries genetic instructions for the development, functioning, growth and reproduction of all known organisms and many viruses. DNA and ribonucleic acid (RNA) are nucleic acids. Alongside proteins, lipids and complex carbohydrates (polysaccharides), nucleic acids are one of the four major types of macromolecules that are essential for all known forms of life. The two DNA strands are known as polynucleotides as they are composed of simpler monomeric units called nucleotides. Each nucleotide is composed of one of four nitrogen-containing nucleobases (cytosine [C], guanine [G], adenine [A] or thymine [T]), a sugar called deoxyribose, and a phosphate group. The nucleotides are joined to one another in a chain by covalent bonds (known as the phosphodiester linkage) between the sugar of one nucleotide and the phosphate of the next, resulting in an alternating sugar-phosphate backbone. The nitrogenous bases of the two separate polynucleotide strands are bound together, according to base pairing rules (A with T and C with G), with hydrogen bonds to make double-stranded DNA. The complementary nitrogenous bases are divided into two groups, the single-ringed pyrimidines and the double-ringed purines. In DNA, the pyrimidines are thymine and cytosine; the purines are adenine and guanine. Both strands of double-stranded DNA store the same biological information. This information is replicated when the two strands separate. The two strands of DNA run in opposite directions to each other and are thus antiparallel. Attached to each sugar is one of four types of nucleobases (or bases). It is the sequence of these four nucleobases along the backbone that encodes genetic information. RNA strands are created using DNA strands as a template in a process called transcription, where DNA bases are exchanged for their corresponding bases except in the case of thymine (T), for which RNA substitutes uracil (U). Under the genetic code, these RNA strands specify the sequence of amino acids within proteins in a process called translation. Within eukaryotic cells, DNA is organized into long structures called chromosomes. Before typical cell division, these chromosomes are duplicated in the process of DNA replication, providing a complete set of chromosomes for each daughter cell. Eukaryotic organisms (animals, plants, fungi and protists) store most of their DNA inside the cell nucleus as nuclear DNA, and some in the mitochondria as mitochondrial DNA or in chloroplasts as chloroplast DNA. In contrast, prokaryotes (bacteria and archaea) store their DNA only in the cytoplasm, in circular chromosomes. Within eukaryotic chromosomes, chromatin proteins, such as histones, compact and organize DNA. These compacting structures guide the interactions between DNA and other proteins, helping control which parts of the DNA are transcribed.

Nuclear chemistry is the sub-field of chemistry dealing with radioactivity, nuclear processes, and transformations in the nuclei of atoms, such as nuclear transmutation and nuclear properties. It is the chemistry of radioactive elements such as the actinides, radium and radon together with the chemistry associated with equipment (such as nuclear reactors) which are designed to perform nuclear processes. This includes the corrosion of surfaces and the behavior under conditions of both normal and abnormal operation (such as during an accident). An important area is the behavior of objects and materials after being placed into a nuclear waste storage or disposal site. It includes the study of the chemical effects resulting from the absorption of radiation within living animals, plants, and other materials. The radiation chemistry controls much of radiation biology as radiation has an effect on living things at the molecular scale. To explain it another way, the radiation alters the biochemicals within an organism, the alteration of the bio-molecules then changes the chemistry which occurs within the organism; this change in chemistry then can lead to a biological outcome. As a result, nuclear chemistry greatly assists the understanding of medical treatments (such as cancer radiotherapy) and has enabled these treatments to improve. It includes the study of the production and use of radioactive sources for a range of processes. These include radiotherapy in medical applications; the use of radioactive tracers within industry, science and the environment, and the use of radiation to modify materials such as polymers. It also includes the study and use of nuclear processes in non-radioactive areas of human activity. For instance, nuclear magnetic resonance (NMR) spectroscopy is commonly used in synthetic organic chemistry and physical chemistry and for structural analysis in macro-molecular chemistry.

A carbohydrate () is a sugar (saccharide) or a sugar derivative. For the simplest carbohydrates, the carbon-to-hydrogen-to-oxygen atomic ratio is 1:2:1, i.e. they are often represented by the empirical formula (CH2O)n. Together with amino acids, fats, and nucleic acids, the carbohydrates are one of the major families of biomolecules. Carbohydrates perform numerous roles in living organisms. Polysaccharides serve as an energy store (e.g., starch and glycogen) and as structural components (e.g., cellulose in plants and chitin in arthropods and fungi). The 5-carbon monosaccharide ribose is an important component of coenzymes (e.g., ATP, FAD and NAD) and the backbone of the genetic molecule known as RNA. The related deoxyribose is a component of DNA. Saccharides and their derivatives play key roles in the immune system, fertilization, preventing pathogenesis, blood clotting, and development. Carbohydrates are central to nutrition and are found in a wide variety of natural and processed foods. Starch is a polysaccharide and is abundant in cereals (wheat, maize, rice), potatoes, and processed food based on cereal flour, such as bread, pizza or pasta. Sugars appear in the human diet mainly as table sugar (sucrose, extracted from sugarcane or sugar beets), lactose (abundant in milk), glucose and fructose, both of which occur naturally in honey, many fruits, and some vegetables. Table sugar, milk, or honey is often added to drinks and many prepared foods such as jam, biscuits, scones and cakes.

Lipids are a broad group of organic compounds that include fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids include storing energy, signaling, and acting as structural components of cell membranes. Lipids have applications in the cosmetic and food industries, and in nanotechnology. Lipids are broadly defined as hydrophobic or amphiphilic small molecules; the amphiphilic nature of some lipids allows them to form structures such as vesicles, multilamellar/unilamellar liposomes, or membranes in an aqueous environment. Biological lipids originate entirely or in part from two distinct types of biochemical subunits or "building-blocks": ketoacyl and isoprene groups. Using this approach, lipids may be divided into eight categories: fatty acyls, glycerolipids, glycerophospholipids, sphingolipids, saccharolipids, and polyketides (derived from condensation of ketoacyl subunits); and sterol lipids and prenol lipids (derived from condensation of isoprene subunits). Although the term lipid is sometimes used as a synonym for fats, fats are a subgroup of lipids called triglycerides. Lipids also encompass molecules such as fatty acids and their derivatives (including tri-, di-, and monoglycerides, and phospholipids), as well as other sterol-containing metabolites such as cholesterol. Although humans and other mammals use various biosynthetic pathways both to break down and to synthesize lipids, some essential lipids cannot be made this way and must be obtained from the diet.

Hydrogen is a chemical element; it has the symbol H and atomic number 1. It is the lightest and most abundant chemical element in the universe, constituting about 75% of all normal matter. Under standard conditions, hydrogen is a gas of diatomic molecules with the formula H2, called dihydrogen, or sometimes hydrogen gas, molecular hydrogen, or simply hydrogen. Dihydrogen is colorless, odorless, non-toxic, and highly combustible. Stars, including the Sun, mainly consist of hydrogen in a plasma state, while on Earth, hydrogen is found as the gas H2 (dihydrogen) and in molecules, such as in water and organic compounds. The most common isotope of hydrogen, 1H, consists of one proton, one electron, and no neutrons. Hydrogen gas was first produced artificially in the 17th century by the reaction of acids with metals. Henry Cavendish, in 1766–1781, identified hydrogen gas as a distinct substance and discovered its property of producing water when burned: this is the origin of hydrogen's name, which means 'water-former' (from Ancient Greek: ὕδωρ, romanized: húdōr, lit. 'water', and γεννάω, gennáō, 'I bring forth'). Understanding the colors of light absorbed and emitted by hydrogen was a crucial part of the development of quantum mechanics. Hydrogen, typically nonmetallic except under extreme pressure, readily forms covalent bonds with most nonmetals, contributing to the formation of compounds like water and various organic substances. Its role is crucial in acid–base reactions, which mainly involve proton exchange among soluble molecules. In ionic compounds, hydrogen can take the form of either a negatively-charged anion, where it is known as hydride, or as a positively charged cation, H+, hydron. Although tightly bonded to water molecules, hydrons strongly affect the behavior of aqueous solutions, as reflected in the importance of pH. Hydride, on the other hand, is rarely observed because it tends to deprotonate solvents, yielding H2. In the early universe, neutral hydrogen atoms formed about 370,000 years after the Big Bang as the universe expanded and plasma had cooled enough for electrons to remain bound to protons. After stars began to form, most of the hydrogen in the intergalactic medium was re-ionized. Nearly all hydrogen production is done by transforming fossil fuels, particularly steam reforming of natural gas. It can also be produced from water or saline by electrolysis, but this process is more expensive. Its main industrial uses include fossil fuel processing and ammonia production for fertilizer. Emerging uses for hydrogen include the use of fuel cells to generate electricity.

Inorganic chemistry deals with synthesis and behavior of inorganic and organometallic compounds. This field covers chemical compounds that are not carbon-based, which are the subjects of organic chemistry. The distinction between the two disciplines is far from absolute, as there is much overlap in the subdiscipline of organometallic chemistry. It has applications in every aspect of the chemical industry, including catalysis, materials science, pigments, surfactants, coatings, medications, fuels, and agriculture.

Atoms are the basic particles of the chemical elements and the fundamental building blocks of matter. An atom consists of a nucleus of protons and generally neutrons, surrounded by an electromagnetically bound swarm of electrons. The chemical elements are distinguished from each other by the number of protons that are in their atoms. For example, any atom that contains 11 protons is sodium, and any atom that contains 29 protons is copper. Atoms with the same number of protons but a different number of neutrons are called isotopes of the same element. Atoms are extremely small, typically around 100 picometers across. A human hair is about a million carbon atoms wide. Atoms are smaller than the shortest wavelength of visible light, which means humans cannot see atoms with conventional microscopes. They are so small that accurately predicting their behavior using classical physics is not possible due to quantum effects. More than 99.94% of an atom's mass is in the nucleus. Protons have a positive electric charge and neutrons have no charge, so the nucleus is positively charged. The electrons are negatively charged, and this opposing charge is what binds them to the nucleus. If the numbers of protons and electrons are equal, as they normally are, then the atom is electrically neutral as a whole. A charged atom is called an ion. If an atom has more electrons than protons, then it has an overall negative charge and is called a negative ion (or anion). Conversely, if it has more protons than electrons, it has a positive charge and is called a positive ion (or cation). The electrons of an atom are attracted to the protons in an atomic nucleus by the electromagnetic force. The protons and neutrons in the nucleus are attracted to each other by the nuclear force. This force is usually stronger than the electromagnetic force that repels the positively charged protons from one another. Under certain circumstances, the repelling electromagnetic force becomes stronger than the nuclear force. In this case, the nucleus splits and leaves behind different elements. This is a form of nuclear decay. Atoms can attach to one or more other atoms by chemical bonds to form chemical compounds such as molecules or crystals. The ability of atoms to attach and detach from each other is responsible for most of the physical changes observed in nature. Chemistry is the science that studies these changes.

A chemical bond is the association of atoms or ions to form molecules, crystals, and other structures. The bond may result from the electrostatic force between oppositely charged ions, as in ionic bonds; the sharing of electrons, as in covalent bonds; or some combination of these effects. Chemical bonds are described as having different strengths: there are "strong bonds" or "primary bonds" such as covalent, ionic and metallic bonds, and "weak bonds" or "secondary bonds" such as dipole–dipole interactions, the London dispersion force, and hydrogen bonds. Since opposite electric charges attract, the negatively charged electrons surrounding the nucleus and the positively charged protons within a nucleus attract each other. Electrons shared between two nuclei will be attracted to both of them. "Constructive quantum mechanical wavefunction interference" stabilizes the paired nuclei (see Theories of chemical bonding). Bonded nuclei maintain an optimal distance (the bond distance) that balances attractive and repulsive effects, as explained quantitatively by quantum theory. The atoms in molecules, crystals, metals and other forms of matter are held together by chemical bonds, which determine the structure and properties of matter. All bonds can be described by quantum theory, but, in practice, simplified rules and other theories allow chemists to predict the strength, directionality, and polarity of bonds. The octet rule and VSEPR theory are examples. More sophisticated theories are valence bond theory, which includes orbital hybridization and resonance, and molecular orbital theory, which includes the linear combination of atomic orbitals and ligand field theory. Electrostatics is used to describe bond polarities and the effects they have on chemical substances.

A molecule is a group of two or more atoms that are held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions that satisfy this criterion. In quantum physics, organic chemistry, and biochemistry, the distinction from ions is dropped and molecule is often used when referring to polyatomic ions. A molecule may be homonuclear, that is, it consists of atoms of one chemical element, e.g. two atoms in the oxygen molecule (O2); or it may be heteronuclear, a chemical compound composed of more than one element, e.g. water (two hydrogen atoms and one oxygen atom; H2O). In the kinetic theory of gases, the term molecule is often used for any gaseous particle regardless of its composition. This relaxes the requirement that a molecule contains two or more atoms, since the noble gases are individual atoms. Atoms and complexes connected by non-covalent interactions, such as hydrogen bonds or ionic bonds, are typically not considered single molecules. Concepts similar to molecules have been discussed since ancient times, but modern investigation into the nature of molecules and their bonds began in the 17th century. Refined over time by scientists such as Robert Boyle, Amedeo Avogadro, Jean Perrin, and Linus Pauling, the study of molecules is today known as molecular physics or molecular chemistry.

Redox ( RED-oks, REE-doks, reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a decrease in the oxidation state. The oxidation and reduction processes occur simultaneously in the chemical reaction. There are two classes of redox reactions: Electron transfer – Only one (usually) electron flows from the atom, ion, or molecule being oxidized to the atom, ion, or molecule that is reduced. This type of redox reaction is often discussed in terms of redox couples and electrode potentials. Atom transfer – An atom transfers from one substrate to another. For example, in the rusting of iron, the oxidation state of iron atoms increases as the iron converts to an oxide, and simultaneously, the oxidation state of oxygen decreases as it accepts electrons released by the iron. Although oxidation reactions are commonly associated with forming oxides, other chemical species can serve the same function. In hydrogenation, bonds like C=C are reduced by transfer of hydrogen atoms.

Distillation, also classical distillation, is the process of separating the component substances of a liquid mixture of two or more chemically discrete substances by selective boiling of the mixture and the condensation of the vapors in a still. Distillation can operate over a wide range of pressures from 0.14 bar (e.g., ethylbenzene/styrene) to nearly 21 bar (e.g., propylene/propane) and is capable of separating feeds with high volumetric flowrates and various components that cover a range of relative volatilities from only 1.17 (o-xylene/m-xylene) to 81.2 (water/ethylene glycol). Distillation provides a convenient and time-tested solution to separate a diversity of chemicals in a continuous manner with high purity. However, distillation has an enormous environmental footprint, resulting in the consumption of approximately 25% of all industrial energy use. The key issue is that distillation operates based on phase changes, and this separation mechanism requires vast energy inputs. Dry distillation (thermolysis and pyrolysis) is the heating of solid materials to produce gases that condense either into fluid products or into solid products. The term dry distillation includes the separation processes of destructive distillation and of chemical cracking, breaking down large hydrocarbon molecules into smaller hydrocarbon molecules. Moreover, a partial distillation results in partial separations of the mixture's components, which process yields nearly pure components; partial distillation also realizes partial separations of the mixture to increase the concentrations of selected components. In either method, the separation process of distillation exploits the differences in the relative volatility of the component substances of the heated mixture. In the industrial applications of classical distillation, the term distillation is used as a unit of operation that identifies and denotes a process of physical separation, not a chemical reaction; thus an industrial installation that produces distilled beverages, is a distillery of alcohol. These are some applications of the distillation process: Distilling fermented products to yield alcoholic beverages with a high content by volume of ethyl alcohol. Desalination to produce potable water and for medico-industrial applications. Crude oil stabilisation, a partial distillation to reduce the vapor pressure of crude oil, which thus is safe to store and to transport, and thereby reduces the volume of atmospheric emissions of volatile hydrocarbons. Fractional distillation is used in the midstream operations of an oil refinery for producing fuels and chemical raw materials for livestock feed. Cryogenic Air separation into the component gases — oxygen, nitrogen, and argon — for use as industrial gases. Chemical synthesis to separate impurities and unreacted materials.

The September 11 attacks, colloquially known as 9/11, were a coordinated series of suicide attacks perpetrated by the Islamic terrorist organization al-Qaeda against the United States in 2001. Nineteen terrorists hijacked four airliners, then flew one into each of the Twin Towers at the World Trade Center in New York City. The third plane crashed into the Pentagon, the headquarters of the United States Department of Defense, in Arlington County, Virginia. The fourth plane crashed in a rural Pennsylvania field during a passenger revolt. In response to the attacks, the United States launched the global war on terror, seeking to eliminate hostile groups deemed terrorist organizations, and the governments purported to support them. This foreign policy agenda was conducted over the next two decades.Ringleader and hijacker Mohamed Atta flew American Airlines Flight 11 into the North Tower of the World Trade Center complex at 8:46 a.m. At 9:03 a.m., hijacker Marwan al-Shehhi flew United Airlines Flight 175 into the South Tower. Both collapsed within an hour and forty-two minutes, destroying the remaining five structures in the complex. Hijacker Hani Hanjour flew American Airlines Flight 77 into the Pentagon at 9:37 a.m., causing a partial collapse. United Airlines Flight 93, flown by hijacker Ziad Jarrah, was believed to target either the United States Capitol or the White House. Alerted to the previous attacks, the passengers revolted and the hijackers crashed the aircraft into a field near Shanksville, Pennsylvania, at 10:03 a.m. The Federal Aviation Administration ordered a ground stop for all traffic in U.S. airspace, requiring all airborne aircraft to return to their point of origin or divert to Canada. The actions undertaken in Canada to support incoming aircraft and their occupants were collectively titled Operation Yellow Ribbon. That evening, the Central Intelligence Agency informed President George W. Bush that its Counterterrorism Center had identified the attacks as having been the work of al-Qaeda under Osama bin Laden. The United States responded by launching the war on terror and invading Afghanistan. NATO's invocation of Article 5 of the North Atlantic Treaty—its only usage to date—called upon allies to fight al-Qaeda. As U.S. and allied invasion forces swept through Afghanistan, bin Laden eluded them. He denied any involvement until 2004, when excerpts of a taped statement in which he accepted responsibility for the attacks were released. Al-Qaeda's cited motivations included U.S. support of Israel, the presence of U.S. military bases in Saudi Arabia and sanctions against Iraq. The nearly decade-long manhunt for bin Laden concluded in May 2011, when he was killed during a U.S. military raid in Abbottabad, Pakistan. The war in Afghanistan continued for another eight years. The attacks killed 2,977 people, injured thousands more, and gave rise to long-term health consequences, while causing at least US$10 billion in infrastructure and property damage. It remains the deadliest terrorist attack in history, as well as the deadliest incident for firefighters and law enforcement personnel in American history, killing 343 and 72 members, respectively. The crashes of Flight 11 and Flight 175 were the deadliest aviation disasters of all time, and the collision of Flight 77 with the Pentagon resulted in the fourth-highest number of ground fatalities in a plane crash in history. The destruction of the World Trade Center and its environs seriously harmed the U.S. economy and induced global market shocks. Many other countries strengthened anti-terrorism legislation and expanded their powers of law enforcement and intelligence agencies. The total number of deaths caused by the attacks, combined with the death tolls from the conflicts they directly incited, has been estimated by the Costs of War Project to be more than 4.5 million. Cleanup of the World Trade Center site (colloquially known as "Ground Zero") was completed in May 2002, while the Pentagon was repaired within a year. After delays in the design of a replacement complex, six new buildings were planned to replace the lost towers at the World Trade Center site, along with a museum and memorial dedicated to those who were killed or injured in the attacks. The tallest building, One World Trade Center, began construction in 2006, and opened in 2014. Memorials to the attacks include the National September 11 Memorial & Museum in New York City, the Pentagon Memorial in Arlington County, Virginia, and the Flight 93 National Memorial at the Pennsylvania crash site.

The assassination of Archduke Franz Ferdinand, also called the Attack in Sarajevo or Sarajevo incident occurred on 28 June 1914 and was one of the key events that led to World War I. Archduke Franz Ferdinand of Austria, heir presumptive to the Austro-Hungarian throne, and his wife, Sophie, Duchess of Hohenberg, were assassinated by Bosnian Serb student Gavrilo Princip. They were shot at close range while being driven through Sarajevo, the provincial capital of Bosnia and Herzegovina, formally annexed by Austria-Hungary in 1908. Princip was part of a group of six Bosnian assassins together with Muhamed Mehmedbašić, Vaso Čubrilović, Nedeljko Čabrinović, Cvjetko Popović and Trifko Grabež coordinated by Danilo Ilić; all but one were Bosnian Serbs and members of a revolutionary movement later known as Young Bosnia. The political objective of the assassination was to free Bosnia and Herzegovina of Austro-Hungarian rule and establish a common South Slav ("Yugoslav") state. The assassination precipitated the July Crisis, which led to Austria-Hungary declaring war on Serbia and the start of World War I. The assassination team was helped by the Black Hand, a Serbian secret nationalist group; support came from Dragutin Dimitrijević, chief of the military intelligence section of the Serbian general staff, as well as from Major Vojislav Tankosić and Rade Malobabić, a Serbian intelligence agent. Tankosić provided bombs and pistols to the assassins and trained them in their use. The assassins were given access to the same clandestine network of safe-houses and agents that Malobabić used for the infiltration of weapons and operatives into Austria-Hungary. The assassins and key members of the clandestine network were tried in Sarajevo in October 1914. In total twenty-five people were indicted. Five of the six assassins were under 20 years old at the time of the assassination, the exception being Mehmedbašić who was 26 or 27. While the group was dominated by Bosnian Serbs, four were Bosnian Croats, one was a Bosnian Muslim and all of them were Austro-Hungarian citizens; none were from Serbia. Princip was found guilty of murder and high treason, but being too young to be executed, was sentenced to 20 years in jail. The four other attackers also received jail terms. Five of the older prisoners were sentenced to be hanged. Black Hand members were arrested and tried before a Serbian court in Salonika in 1917 on charges of high treason, which were later widely regarded as fabricated. The Black Hand was disbanded, and three of its leaders were executed. Much of what is known about the assassinations comes from these two trials and related records. Princip's legacy was re-evaluated following the breakup of Yugoslavia, and public opinion of him in the successor states is largely divided along ethnic lines.

John F. Kennedy, the 35th president of the United States, was assassinated while riding in a presidential motorcade through Dealey Plaza in Dallas, Texas, on Friday, November 22, 1963. Kennedy was in the vehicle with his wife Jacqueline, Texas governor John Connally, and Connally's wife Nellie, when he was fatally shot from the nearby Texas School Book Depository by Lee Harvey Oswald, a former U.S. Marine. The motorcade rushed to Parkland Memorial Hospital, where Kennedy was pronounced dead about 30 minutes after the shooting; Connally was also wounded in the attack but recovered. Vice President Lyndon B. Johnson was hastily sworn in as president two hours and eight minutes later aboard Air Force One at Dallas Love Field. After the assassination, Oswald returned home to retrieve a pistol; he shot and killed a lone Dallas policeman J. D. Tippit shortly afterwards. Around 70 minutes after Kennedy and Connally were shot, Oswald was apprehended by the Dallas Police Department and charged under Texas state law with the murders of Kennedy and Tippit. Two days later, as live television cameras covered Oswald being moved through the basement of Dallas Police Headquarters, he was fatally shot by Dallas nightclub operator Jack Ruby. Like Kennedy, Oswald was taken to Parkland Memorial Hospital, where he soon died. Ruby was convicted of Oswald's murder, though the decision was overturned on appeal, and Ruby died in prison in 1967 while awaiting a new trial. After a 10-month investigation, the Warren Commission concluded that Oswald assassinated Kennedy, and that there was no evidence that either Oswald or Ruby was part of a conspiracy. In 1967, New Orleans district attorney Jim Garrison brought the only trial for Kennedy's murder against businessman Clay Shaw; Shaw was acquitted. Subsequent federal investigations—such as the Rockefeller Commission and Church Committee—agreed with the Warren Commission's general findings. In its 1979 report, the United States House Select Committee on Assassinations (HSCA) concluded that Kennedy was likely "assassinated as a result of a conspiracy". The HSCA did not identify possible conspirators, but concluded that there was "a high probability that two gunmen fired at [the] President". The HSCA's conclusions were largely based on a police Dictabelt recording, which was subsequently debunked by the U.S. Justice Department. Kennedy's assassination is still the subject of widespread debate and has spawned many conspiracy theories and alternative scenarios; polls have found that a vast majority of Americans believe there was a conspiracy. The assassination left a profound impact and was the first of four major assassinations during the 1960s in the United States, coming two years before the assassination of Malcolm X in 1965, and five years before the assassinations of Martin Luther King Jr. and Kennedy's brother Robert F. Kennedy in 1968. Kennedy was the fourth U.S. president to be assassinated and is the most recent to have died in office.

The Great Depression was a severe global economic downturn from 1929 to 1939. The period was characterized by high rates of unemployment and poverty, drastic reductions in industrial production and international trade, and widespread bank and business failures around the world. The economic contagion began in 1929 in the United States, the largest economy in the world, with the devastating Wall Street crash of 1929 often considered the beginning of the Depression. Among the countries with the most unemployed were the U.S., the United Kingdom, and Germany. The Depression was preceded by a period of industrial growth and social development known as the "Roaring Twenties". Much of the profit generated by the boom was invested in speculation, such as on the stock market, contributing to growing wealth inequality. Banks were subject to minimal regulation, resulting in loose lending and widespread debt. By 1929, declining spending had led to reductions in manufacturing output and rising unemployment. Share values continued to rise until the October 1929 crash, after which the slide continued until July 1932, accompanied by a loss of confidence in the financial system. By 1933, the U.S. unemployment rate had risen to 25%, about one-third of farmers had lost their land, and 9,000 of its 25,000 banks had gone out of business. President Herbert Hoover was unwilling to intervene heavily in the economy, and in 1930 he signed the Smoot–Hawley Tariff Act, which inadvertently worsened the Depression. In the 1932 presidential election, Hoover was defeated by Franklin D. Roosevelt, who from 1933 pursued a set of expansive New Deal programs in order to provide relief and create jobs. In Germany, which depended heavily on U.S. loans, the crisis caused unemployment to rise to nearly 30% and fueled political extremism, paving the way for Adolf Hitler's Nazi Party to rise to power in 1933. Between 1929 and 1932, worldwide gross domestic product (GDP) fell by an estimated 15%; in the U.S., the Depression resulted in a 30% contraction in GDP. Recovery varied greatly around the world. Some economies, such as the U.S., Germany, and Japan, started to recover by the mid-1930s; others, like France, did not return to pre-shock growth rates until later in the decade. The Depression had devastating economic effects on both wealthy and poor countries: all experienced drops in personal income, prices (deflation), tax revenues, and profits. International trade fell by more than 50%, and unemployment in some countries rose as high as 33%. Cities around the world, especially those dependent on heavy industry, were heavily affected. Construction virtually halted in many countries, and farming communities and rural areas suffered as crop prices fell by up to 60%. Faced with plummeting demand and few job alternatives, areas dependent on primary sector industries suffered the most. The outbreak of World War II in 1939 ended the Depression, as it stimulated factory production, providing jobs for women as militaries absorbed large numbers of young, unemployed men. The precise causes for the Great Depression are disputed. One set of historians, for example, focuses on non-monetary economic causes. Among these, some regard the Wall Street crash itself as the main cause; others consider that the crash was a mere symptom of more general economic trends of the time, which had already been underway in the late 1920s. A contrasting set of views, which rose to prominence in the later part of the 20th century, ascribes a more prominent role to failures of monetary policy. According to those authors, while general economic trends can explain the emergence of the downturn, they fail to account for its severity and longevity; they argue that these were caused by the lack of an adequate response to the crises of liquidity that followed the initial economic shock of 1929 and the subsequent bank failures accompanied by a general collapse of the financial markets.

The Russian Revolution was a period of political and social change in Russia, starting in 1917. This period saw Russia abolish its monarchy and adopt a socialist form of government following two successive revolutions and a civil war. It can be seen as the precursor for other revolutions that occurred in the aftermath of World War I, such as the German Revolution of 1918–1919. The Russian Revolution was a key event of the 20th century. The Russian Revolution was inaugurated with the February Revolution in 1917, in the midst of World War I. With the German Empire inflicting defeats on the front, and increasing logistical problems causing shortages of bread and grain, the Russian Army was losing morale, with large scale mutiny looming. Officials were convinced that if Tsar Nicholas II abdicated, the unrest would subside. Nicholas stepped down on 15 March [O.S. 2 March], ushering in a provisional government led by the Duma (parliament). During the unrest, Soviet councils were formed by locals in Petrograd (now Saint Petersburg) that initially did not oppose the new government; however, the Soviets insisted on their influence in the government and control over militias. By March, Russia had two rival governments. The Provisional Government held state power in military and international affairs, whereas the network of Soviets held domestic power. Critically, the Soviets held the allegiance of the working class, and urban middle class. There were mutinies, protests and strikes. Socialist and other leftist political organizations competed for influence within the Provisional Government and Soviets. Factions included the Mensheviks, Socialist Revolutionaries, Anarchists, and the Bolsheviks, a far-left party led by Vladimir Lenin. The Bolsheviks won popularity with their program promising peace, land, and bread: an end to the war, land for the peasantry, and ending famine. After assuming power, the Provisional Government continued fighting the war in spite of public opposition. Taking advantage, the Bolsheviks and other factions gained popular support to advance the revolution. Responding to discontent in Petrograd, the Provisional Government repressed protestors leading to the July Days. The Bolsheviks merged workers' militias loyal to them into the Red Guards. The volatile situation reached its climax with the October Revolution, a Bolshevik armed insurrection in Petrograd beginning 7 November [O.S. 25 October] that overthrew the Provisional Government. The Bolsheviks established their own government and proclaimed the establishment of the Russian Soviet Federative Socialist Republic (RSFSR). Under pressure from German military offensives, the Bolsheviks relocated the capital to Moscow. The RSFSR began reorganizing the empire into the world's first socialist state, to practice soviet democracy on a national and international scale. Their promise to end Russia's participation in World War I was fulfilled when Bolshevik leaders signed the Treaty of Brest-Litovsk with Germany in March 1918. The Bolsheviks established the Cheka, a secret police and revolutionary security service working to uncover, punish, and eliminate those considered to be "enemies of the people" in campaigns called the Red Terror. Although the Bolsheviks held large support in urban areas, they had foreign and domestic enemies that refused to recognize their government. Russia erupted into a bloody civil war, which pitted the Reds (Bolsheviks), against their enemies, which included nationalist movements, anti-Bolshevik socialist parties, anarchists, monarchists and liberals; the latter two parties strongly supported the Russian White movement which was led mainly by right-leaning officers and seen as fighting for the restoration of the imperial order. The Bolshevik commissar Leon Trotsky began organizing workers' militias loyal to the Bolsheviks into the Red Army. While key events occurred in Moscow and Petrograd, every city in the empire was convulsed, including the provinces of national minorities, and in the rural areas peasants took over and redistributed land. As the war progressed, the RSFSR established Soviet power in Armenia, Azerbaijan, Byelorussia, Georgia, and Ukraine. Wartime cohesion and intervention from foreign powers prompted the RSFSR to begin unifying these nations under one flag and created the Soviet Union. Historians consider the end of the revolutionary period to be in 1922, when the civil war concluded with the defeat of the White Army and separatist factions, leading to mass emigration from Russia. The victorious Bolshevik Party reconstituted itself into the All-Union Communist Party (Bolsheviks) and remained in power for six decades.

On 26 December 2004, at 07:58:53 local time (UTC+7), a Mw 9.2–9.3 earthquake struck with its epicenter off the west coast of Aceh, in northern Sumatra, Indonesia. The undersea megathrust earthquake, known in the scientific community as the Sumatra–Andaman earthquake, was caused by a rupture along the fault boundary between the Burma plate and the Indian plate, and reached a maximum Mercalli intensity of IX in some areas. The earthquake caused a massive tsunami with waves up to 30 m (100 ft) high, referred to by British and Commonwealth media as the Boxing Day tsunami, after the Boxing Day holiday, or the Asian tsunami, which devastated communities along the surrounding coasts of the Indian Ocean, killing an estimated 227,898 people in 14 countries, especially in Aceh in Indonesia, Sri Lanka, Tamil Nadu in India, and Khao Lak in Thailand. The direct result was severe disruption to living conditions and commerce in coastal provinces of these and other surrounding countries. It is the deadliest tsunami in history, the deadliest natural disaster of the 21st century, and one of the deadliest natural disasters in recorded history. It is also the worst natural disaster in the history of Indonesia, the Maldives, Sri Lanka, and Thailand. The earthquake itself is the most powerful earthquake ever recorded in Asia, the strongest of the 21st century, and the second- or third-most powerful globally since modern seismography began in 1900. It had the longest fault rupture ever observed, between 1,200 to 1,300 kilometres (746 to 808 mi), and had the longest duration of faulting ever observed, lasting at least ten minutes. It caused the entire planet to vibrate by as much as 10 mm (0.4 in), and also remotely triggered earthquakes as far away as Alaska. Its epicentre was located between Simeulue and mainland Sumatra. The plight of the affected people and countries prompted a worldwide humanitarian response, with donations totaling more than US$14 billion (equivalent to US$24 billion in 2025 currency).

The Battle of Gettysburg (locally ) (July 1–3, 1863) was fought in the American Civil War between the Union and Confederate armies in and around Gettysburg, Pennsylvania. The battle, won by the Union, is widely considered the Civil War's turning point, leading to an ultimate victory of the Union and the preservation of the nation. The Battle of Gettysburg was the bloodiest battle of both the Civil War and of any battle in American military history up to that time, claiming over 50,000 combined casualties. Union Major General George Meade's Army of the Potomac defeated attacks by Confederate General Robert E. Lee's Army of Northern Virginia, halting Lee's invasion of the North and forcing his retreat. After his success in the Battle of Chancellorsville in Spotsylvania County, Virginia, in May 1863, Lee led his Confederate forces through the Shenandoah Valley to begin the Gettysburg campaign, his second attempt to invade the North. With Lee's army in high spirits, he intended to shift the focus of the summer campaign from war-ravaged Northern Virginia in the hopes of penetrating as far as Harrisburg or Philadelphia, which he hoped would convince northern politicians to end the war. President Abraham Lincoln initially prodded Major General Joseph Hooker into pursuing Lee, then relieved him of command just three days before the Battle of Gettysburg commenced, replacing him with Meade. On July 1, 1863, as Lee's forces moved on Gettysburg in the hopes of destroying the Union army, the two armies encountered each other, and the battle commenced. Low ridges to the northwest of Gettysburg were initially defended by a Union cavalry division under Brigadier General John Buford, soon reinforced by two corps of Union infantry. Two large Confederate corps assaulted them from the northwest and north, however, collapsing the hastily developed Union lines, leading them to retreat through the streets of Gettysburg to the hills just south of the city. On the second day of battle, on July 2, the Union line was laid out in a defensive formation resembling a fishhook. In the late afternoon, Lee launched a heavy assault on the Union's left flank, leading to fierce fighting at Little Round Top, the Wheatfield, Devil's Den, and the Peach Orchard. On the Union's right flank, Confederate demonstrations escalated into full-scale assaults on Culp's Hill and Cemetery Hill. Despite incurring significant losses, Union forces held their lines. On the third day of battle, July 3, fighting resumed on Culp's Hill, and cavalry battles raged to the east and south of Gettysburg. Pickett's Charge featured the main engagement, a Confederate infantry assault of approximately 12,000 Confederate troops, who attacked the center of the Union line at Cemetery Ridge, which was repelled by Union rifle and artillery fire, leading to great Confederate losses. The following day, on the Fourth of July, Lee led his Confederate troops on the torturous retreat from the North. Between 46,000 and 51,000 soldiers from both armies were casualties in the three-day Battle of Gettysburg, the most in any battle in American history up to that time. On November 19, Lincoln traveled to Gettysburg, where he spoke at a ceremony dedicating Gettysburg National Cemetery, which honored the fallen Union soldiers and redefined the purpose of the Civil War in his famed Gettysburg Address, a 271-word speech that has endured as one of the most famous in American history.

The partition of India in 1947 was the division of British India into two independent dominion states, the Union of India and Dominion of Pakistan. The Union of India is today the Republic of India, and the Dominion of Pakistan is the Islamic Republic of Pakistan and the People's Republic of Bangladesh. The partition involved the division of two provinces, Bengal and the Punjab, based on district-wide non-Muslim (mostly Hindu and Sikh) or Muslim majorities. It also involved the division of the British Indian Army, the Royal Indian Navy, the Indian Civil Service, the railways, and the central treasury, between the two new dominions. The partition was set forth in the Indian Independence Act 1947 and resulted in the end of the British Raj, or Crown rule in India. The two self-governing countries of India and Pakistan legally came into existence at midnight on 14–15 August 1947. With the partition of British India and withdrawal of the British from the Indian subcontinent, the Indian Independence Act provided that the princely states were released from their subsidiary alliances and other obligations to the British, while the British withdrew from their obligations to the states, leaving the rulers to decide whether to accede to India or Pakistan or to remain independent outside both. Sardar Patel said in a speech in January 1948, "On the lapse of Paramountcy, every Indian State became a separate independent entity." The political integration of the princely states into the two new dominions was begun by many accessions in August 1947, but mostly it followed on later. Critically, the rulers of the states of Hyderabad, Jammu and Kashmir, and others, chose independence. The partition displaced between 12 and 20 million people along religious lines, creating overwhelming refugee crises associated with the mass migration and population transfer that occurred across the newly constituted dominions; there was large-scale violence, with disputed estimates of the loss of life before, at the time of, or following the partition. As of 2009, the estimates still varied between two hundred thousand and two million. The violent nature of the partition created an atmosphere of hostility and suspicion between India and Pakistan that plagues Indo-Pakistani relations to the present. The term partition of India does not cover the earlier separations of Burma (now Myanmar) and Ceylon (now Sri Lanka) from the administration of British India. It does not cover the incorporation of the enclaves of French India into India during the period 1947–1954, nor the annexation of Goa and other districts of Portuguese India by India in 1961. Other contemporaneous political entities in the region in 1947, such as Sikkim, Bhutan, Nepal, and the Maldives, were unaffected by the partition. The secession of Bangladesh from Pakistan in 1971 was a later partition of Pakistan, and not of India.

The Normandy landings were the landing operations and associated airborne operations on 6 June 1944 of the Allied invasion of Normandy in Operation Overlord during the Second World War. Codenamed Operation Neptune and often referred to as D-Day (after the military term), it is the largest seaborne invasion in history. The operation began the liberation of France and the rest of Western Europe, and laid the foundations for the Allied victory on the Western Front. Planning for the operation began in 1943. In the months leading up to the invasion, the Allies conducted a substantial military deception, codenamed Operation Bodyguard, to mislead the Germans as to the date and location of the main Allied landings. The weather on the day selected for D-Day was not ideal, and the operation had to be delayed 24 hours; a further postponement would have meant a delay of at least two weeks, as the planners had requirements for the phase of the moon, the tides, and time of day, that meant only a few days each month were deemed suitable. German leader Adolf Hitler placed Field Marshal Erwin Rommel in command of German forces and developing fortifications along the Atlantic Wall in anticipation of an invasion. US president Franklin D. Roosevelt placed Major General Dwight D. Eisenhower in command of Allied forces. The invasion began shortly after midnight on the morning of 6 June with extensive aerial and naval bombardment as well as an airborne assault—the landing of 24,000 American, British, and Canadian airborne troops. The early morning aerial assault was soon followed by Allied amphibious landings on the coast of France c. 06:30. The target 80-kilometre (50 mi) stretch of the Normandy coast was divided into five sectors: Utah, Omaha, Gold, Juno, and Sword. Strong winds blew the landing craft east of their intended positions, particularly at Utah and Omaha. The men landed under heavy fire from gun emplacements overlooking the beaches, and the shore was mined and covered with obstacles such as wooden stakes, metal tripods, and barbed wire, making the work of the beach-clearing teams difficult and dangerous. The highest number of casualties was at Omaha, with its high cliffs. At Gold, Juno, and Sword, several fortified towns were cleared in house-to-house fighting, and two major gun emplacements at Gold were disabled using specialised tanks. The Allies were able to establish beachheads at each of the five landing sites on the first day, but Carentan, Saint-Lô, and Bayeux remained in German hands. Caen, a major objective, was not captured until 21 July. Only two of the beaches (Juno and Gold) were linked on the first day, and all five beachheads were not connected until 12 June. German casualties on D-Day have been estimated at 4,000 to 9,000 men. Allied casualties were at least 10,000, with 4,414 confirmed dead.

The Boston Tea Party was an act of protest on December 16, 1773 during the American Revolution. Initiated by the Sons of Liberty in Boston, the capital of Massachusetts, one of the Thirteen Colonies of British America, it escalated hostilities between Great Britain and the Patriots, who opposed British policy towards its American colonies. Less than two years later, on April 19, 1775, the Battles of Lexington and Concord, also in Massachusetts, launched the eight-year American Revolutionary War, which resulted in the independence of the colonies as the United States. The source of the protestors' anger was the passage of the Tea Act by the Parliament of Great Britain on May 10, 1773, which allowed the East India Company (EIC) to sell Chinese tea in the colonies without paying taxes apart from those imposed by the Townshend Acts. The Sons of Liberty strongly opposed both the Tea Act and Townshend Acts, which they saw as a violation of their "rights as Englishmen" to no taxation without representation. Disguised as Native Americans, on the night of December 16 members of Sons of Liberty boarded the Dartmouth, a merchantman that had docked in Boston carrying a major shipment of EIC tea, and set about throwing 340 or 342 chests of the tea into Boston Harbor. The British government considered the protest an act of treason and responded with several punitive measures. Nine days later, on December 25, during the Philadelphia Tea Party, a group of Patriots forced the merchantmen Polly, which was transporting a shipment of tea, to return to England without unloading her cargo. Parliament responded in 1774 with the Intolerable Acts, which, among other provisions, ended local self-government in Massachusetts and closed Boston's commerce. Colonists throughout the Thirteen Colonies responded to the Intolerable Acts with additional acts of protest, and by convening the First Continental Congress in Philadelphia, which sent a Petition to the King for repeal of the acts and coordinated colonial resistance to them, culminating in the October 1774 Continental Association.

The Berlin Wall fell on 9 November 1989 during the Peaceful Revolution, marking the beginning of the destruction of the figurative Iron Curtain, as East Berlin transit restrictions were overwhelmed and discarded. Sections of the wall were breached, and planned deconstruction began the following June. It was one of the series of events that started the fall of communism in Central and Eastern Europe. The fall of the inner German border took place shortly afterward. An end to the Cold War was declared at the Malta Summit in early December, and German reunification took place in October the following year.

The Fall of Constantinople, also known as the Conquest of Constantinople, was the capture of Constantinople, the capital city of the Byzantine Empire, by the Ottoman Empire. The city was captured on 29 May 1453 as part of the culmination of a 53-day siege which had begun on 6 April. The attacking Ottoman Army, which significantly outnumbered Constantinople's defenders, was commanded by Sultan Mehmed II (later nicknamed "the Conqueror"), while the Byzantine army was led by Emperor Constantine XI Palaiologos. After conquering the city, Mehmed II made Constantinople the new Ottoman capital, replacing Adrianople. The fall of Constantinople and of the Byzantine Empire was a watershed moment of the Late Middle Ages, marking the effective end of the Roman Empire, a state which began in roughly 27 BC and had lasted nearly 1,500 years. For many modern historians, the fall of Constantinople marks the end of the medieval period and the beginning of the early modern period. The city's fall also stood as a turning point in military history. Since ancient times, cities and castles had depended upon ramparts and walls to repel invaders. The walls of Constantinople, especially the Theodosian walls, protected Constantinople from attack for 800 years and were noted as some of the most advanced defensive systems in the world at the time. However, these fortifications were overcome by Ottoman infantry with the support of gunpowder, specifically from cannons and bombards, heralding a change in siege warfare. The Ottoman cannons repeatedly fired massive cannonballs weighing 500 kilograms (1,100 lb) over 1.5 kilometres (0.93 mi) which created gaps in the Theodosian walls for the Ottoman siege.

Hurricane Katrina was an extremely powerful and catastrophic tropical cyclone that killed 1,392 people and caused damage estimated at $125 billion, particularly in and around the city of New Orleans, in late August 2005. It is tied with Hurricane Harvey as being the costliest tropical cyclone in the Atlantic basin. Katrina was the twelfth tropical cyclone, the fifth hurricane, the third major hurricane, and the second Category 5 hurricane of the 2005 Atlantic hurricane season. It was also the fourth-most intense Atlantic hurricane to make landfall in the contiguous United States, as measured by barometric pressure. Katrina formed on August 23, 2005, with the merger of a tropical wave and the remnants of a tropical depression. After briefly strengthening to a tropical storm over south Florida, Katrina entered the Gulf of Mexico on August 26 and rapidly intensified to a Category 5 hurricane before weakening to a Category 3 at its landfall on August 29 near Buras-Triumph, Louisiana. Eighty percent of New Orleans, as well as large areas in neighboring parishes, were flooded. It is estimated that about 100,000 to 150,000 people remained in the City of New Orleans, despite mandatory evacuation orders. The flooding prompted a massive national and international response effort, including federal, local, and private rescue operations. The largest loss of life was due to flooding caused by engineering flaws in the federally built hurricane protection system, particularly the levees around New Orleans. Investigators concluded that the U.S. Army Corps of Engineers, tasked by Congress in the Flood Control Act of 1965 to design and build the region's hurricane protection, was responsible for the breached floodwalls. Later, a federal appeals court ruled that the Army Corps, despite being responsible, could not be held financially liable due to the Flood Control Act of 1928. The emergency response from federal, state, and local governments was widely criticized, leading to the resignation of Federal Emergency Management Agency (FEMA) director Michael D. Brown and New Orleans Police Department (NOPD) superintendent Eddie Compass. Many other government officials faced criticism for their responses, especially New Orleans mayor Ray Nagin, Louisiana governor Kathleen Blanco, and President George W. Bush. However, several agencies, such as the United States Coast Guard (USCG), National Hurricane Center (NHC), and National Weather Service (NWS), were commended for their actions, with the NHC being particularly praised for its accurate forecasts well in advance. The destruction and loss of life caused by the storm prompted the name Katrina to be retired by the World Meteorological Organization in April 2006. On January 4, 2023, the NHC updated the Katrina fatality data on the basis of a 2014 report, which reduced the total number from an estimated 1,833 to 1,392.

The RMS Titanic sank in the North Atlantic Ocean on 15 April 1912. The largest ocean liner in service at the time, Titanic was four days into her maiden voyage from Southampton, England, to New York City, United States, with over 2,200 people on board when she struck an iceberg at 23:40 (ship's time) on 14 April. She sank two hours and forty minutes later at 02:20 ship's time (05:18 GMT) on 15 April, resulting in the deaths of up to 1,635 people, making it one of the deadliest peacetime maritime disasters in history. Titanic's wireless operators received seven warnings of sea ice leading up to the night of 14 April, but was travelling at a speed of roughly 22 knots (41 km/h; 25 mph) when her lookouts sighted the iceberg. Unable to turn quickly enough, the ship suffered a glancing blow that buckled the steel plates covering her starboard side and opened six of her sixteen compartments to the sea. Titanic had been designed to stay afloat with up to four of her forward compartments flooded, and the crew used distress rockets and wireless messages to attract rescue vessels as the passengers were put into lifeboats. In accordance with existing practice, the Titanic's lifeboat system was not designed to hold everyone on board simultaneously, but rather to ferry passengers to nearby rescue vessels; therefore, with the ship sinking rapidly and help still hours away, there was no safe refuge for many of the passengers and crew, as the ship was equipped with only twenty lifeboats, including four collapsible lifeboats. Poor preparation for, and management of, the evacuation meant many boats were launched before they were completely full. Titanic sank with over a thousand passengers and crew still on board. Almost all of those who ended up in the water died within minutes due to the effects of cold shock and hypothermia. RMS Carpathia arrived about an hour and a half after the sinking and rescued all of the 710 survivors by 09:15 on 15 April. The disaster shocked the world and caused widespread outrage over the shortage of lifeboats and lack of vigilance given to ice warnings the ship had received. Further outrage was aimed at SS Californian who, despite being the closest ship, missed the Titanic's distress calls due to having only one wireless operator aboard. Subsequent inquiries recommended sweeping changes to maritime regulations, including leading to the establishment of the International Convention for the Safety of Life at Sea (SOLAS) in 1914 which still governs maritime safety today.

On 6 and 9 August 1945, the United States detonated two atomic bombs over the Japanese cities of Hiroshima and Nagasaki, respectively, during the final days of World War II. The aerial bombings killed 150,000 to 246,000 people, most of whom were civilians, and remain the first and only uses of nuclear weapons in an armed conflict. In the final year of World War II, the Allies prepared for a costly invasion of the Japanese mainland. This undertaking was preceded by a conventional bombing and firebombing campaign that devastated 64 Japanese cities, including an operation on Tokyo. The war in Europe concluded when Germany surrendered on 8 May 1945, and the Allies turned their full attention to the Pacific War. By July 1945, the Allies' Manhattan Project had produced two types of atomic bombs: "Little Boy", an enriched uranium gun-type fission weapon, and "Fat Man", a plutonium implosion-type nuclear weapon. The 509th Composite Group of the U.S. Army Air Forces was trained and equipped with the specialized Silverplate version of the Boeing B-29 Superfortress, and deployed to Tinian in the Mariana Islands. The Allies called for the unconditional surrender of the Imperial Japanese Armed Forces in the Potsdam Declaration on 26 July 1945, the alternative being "prompt and utter destruction". The Japanese government ignored the ultimatum. Japan announced its surrender to the Allies on 15 August, six days after the bombing of Nagasaki and the Soviet Union's declaration of war against Japan and invasion of Manchuria. The Japanese government signed an instrument of surrender on 2 September, ending the war. The consent of the United Kingdom was obtained for the bombing, as was required by the Quebec Agreement, and orders were issued on 25 July by General Thomas T. Handy, the acting chief of staff of the U.S. Army, for atomic bombs to be used on Hiroshima, Kokura, Niigata, and Nagasaki. These targets were chosen because they were large urban areas that also held significant military facilities. On 6 August, a Little Boy was dropped on Hiroshima. Three days later, a Fat Man was dropped on Nagasaki. Over the next two to four months, the effects of the atomic bombings killed 90,000 to 166,000 people in Hiroshima and 60,000 to 80,000 people in Nagasaki; roughly half the deaths occurred on the first day. For months afterward, many people continued to die from the effects of burns, radiation sickness, and other injuries, compounded by illness and malnutrition. Despite Hiroshima's sizable military garrison, estimated at 24,000 troops, some 90% of the dead were civilians. Scholars have extensively studied the effects of the bombings on the social and political character of subsequent world history and popular culture, and there is still much debate concerning the ethical and legal justification for the bombings as well as their ramifications of geopolitics especially with the context of the Cold War. Supporters argue that the atomic bombings were necessary to bring an end to the war with minimal casualties and ultimately prevented a greater loss of life on both sides, and also assert that the demonstration of atomic weaponry created the Long Peace in the interest of preventing a nuclear war. Conversely, critics argue that the bombings were unnecessary for the war's end and were a war crime, raising moral and ethical implications, and also assert that future use of atomic weaponry is more likely than anticipated and could lead to a nuclear holocaust.

The global COVID-19 pandemic (also known as the coronavirus pandemic), caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), began with an outbreak in Wuhan, China, in December 2019. It spread to other parts of Asia and then worldwide in early 2020. The World Health Organization (WHO) declared the outbreak a public health emergency of international concern (PHEIC) on 30 January 2020 and assessed it as having become a pandemic on 11 March. The WHO declared that the public health emergency caused by COVID-19 had ended in May 2023, while noting that COVID-19 continued to be a global health threat. COVID-19 symptoms range from asymptomatic to deadly, but most commonly include fever, sore throat, nocturnal cough, and fatigue. Transmission of the virus is often through airborne particles. Mutations have produced many strains (variants) with varying degrees of infectivity and virulence. COVID-19 vaccines were developed rapidly and deployed to the general public beginning in December 2020, made available through government and international programmes such as COVAX, aiming to provide vaccine equity. Treatments include novel antiviral drugs and symptom control. Common mitigation measures during the public health emergency included travel restrictions, lockdowns, business restrictions and closures, workplace hazard controls, mask mandates, quarantines, testing systems, and contact tracing of the infected. The pandemic caused severe social and economic disruption around the world, including the largest global recession since the Great Depression. Widespread supply shortages, including food shortages, were caused by supply chain disruptions and panic buying. Reduced human activity led to an unprecedented temporary decrease in pollution. Educational institutions and public areas were partially or fully closed in many jurisdictions, and many events were cancelled or postponed during 2020 and 2021. Telework became much more common for white-collar workers as the pandemic evolved. Misinformation circulated through social media and occasionally through mass media, and political tensions intensified. The pandemic raised issues of racial and geographic discrimination, health equity, and the balance between public health imperatives and individual rights. The disease has continued to circulate since 2023. By 2025, experts generally believed the pandemic to be over, having transitioned into the endemic phase. Different definitions of pandemics lead to different determinations of when they end. As of 28 May 2026, COVID-19 has caused 7,114,238 confirmed deaths and 18.5 to 35.2 million estimated deaths. The pandemic was the fifth-deadliest pandemic or epidemic in history.

A Moon landing or lunar landing is the arrival of a crewed or robotic spacecraft on the Moon. The first human-made object to touch the Moon was Luna 2 in 1959, and the first crewed mission to land on the Moon was Apollo 11 in 1969. There were six crewed landings between 1969 and 1972 and numerous uncrewed landings. All crewed missions to the Moon were conducted by the Apollo program, with the last departing in December 1972. After Luna 24 in 1976, there were no soft landings—landings without significant damage—on the Moon until Chang'e 3 in 2013. All soft landings took place on the near side of the Moon until Chang'e 4 landed on the far side of the Moon in 2019.

In 79 AD, Mount Vesuvius, a stratovolcano located in the modern-day region of Campania, Italy, erupted, causing one of the deadliest eruptions in history. Vesuvius violently ejected a cloud of super-heated tephra and gases to a height of 33 km (21 mi), ejecting molten rock, pulverized pumice and hot ash at 1.5 million tons per second, ultimately releasing 100,000 times the thermal energy of the atomic bombings of Hiroshima and Nagasaki. The event gives its name to the Vesuvian type of volcanic eruption, characterised by columns of hot gases and ash reaching the stratosphere, although the event also included pyroclastic flows associated with Peléan eruptions. The event destroyed several Roman towns and settlements in the area. Pompeii and Herculaneum, obliterated and buried underneath massive pyroclastic surges and ashfall deposits, are the most famous examples. Archaeological excavations have revealed much of the towns and the lives of the inhabitants, leading to the area becoming Vesuvius National Park and a UNESCO World Heritage Site. The total population of both cities was over 20,000. The remains of over 1,500 people have been found at Pompeii and Herculaneum. The total death toll from the eruption remains unknown. A detailed firsthand account of the eruption is preserved in two letters of Pliny the Younger, who personally witnessed the event from the town of Misenum.

Black Thursday is a term used to refer to typically negative, notable events that have occurred on a Thursday. It has been used in the following cases: 6 February 1851, a devastating day of bushfires in Victoria, Australia 21 June 1877, an execution of 10 suspected leaders of the "Molly Maguires" 8 November 1901 (21 November in the Gregorian calendar), the climax of the gospel riots in Athens. 24 October 1929, start of the Wall Street crash of 1929. 14 October 1943, when the USAAF suffered large losses during bombing in the second Schweinfurt raid during World War II 12 April 1951, during the Korean War, when 25% of the Far East Air Force B-29 bombing force were damaged or destroyed by Soviet MiG-15s in MiG Alley. 21 November 1968, day of protests by students at University of Wisconsin-Oshkosh 12 April 1973, clashes between the police and right-wing demonstrators in Milan resulted in the killing of policeman Antonio Marino. 30 May 1975, the massacre of about 50 Lebanese Christians in the area of Bashoura in West Beirut. 22 January 1987, the Mendiola massacre, which claimed the lives of 13 protesters in Manila, Philippines 3 September 1987, known as the "Black Thursday of Warsaw Transit", when 15 people died in two separate rail accidents in Warsaw, Poland 24 July 2003, when former Guatemalan president Efraín Ríos Montt's supporters rioted in protest of a court decision barring him from standing in the 2003 general election 30 September 2009, when the Irish government revealed to its people the alleged full cost of bailing out Anglo-Irish Bank 15 November 2018, the Franco-Ontarian jeudi noir when the government of Ontario announced the elimination of several Franco-Ontarian institutions 12 March 2020, Black Thursday stock market crash A massacre during the 2022 Chadian protests "Black Thursday", the week day preceding Black Friday

On 26 April 1986, reactor 4 of the Chernobyl Nuclear Power Plant, located near Pripyat, Ukrainian SSR, Soviet Union (later Ukraine), exploded. With dozens of direct casualties and thousands of health complications stemming from the disaster, it is one of only two nuclear accidents rated at the maximum severity on the International Nuclear Event Scale, the other being the 2011 Fukushima nuclear accident. The response involved more than 500,000 personnel and cost an estimated 18 billion rubles (about US$84.5 billion in 2025). It remains the worst nuclear disaster and the most expensive disaster in history, with an estimated cost of US$700 billion. The disaster occurred during a test to simulate cooling the reactor during a serious accident in blackout conditions. The operators carried out the test following an accidental drop in reactor power. Upon shutting down the reactor in those conditions, design flaws led to a dramatic power surge. The reactor components ruptured and lost coolant, and the resulting steam explosions and meltdown destroyed the reactor building. This was followed by a reactor core fire that spread radioactive contaminants across the Soviet Union and Europe. The Soviet government established a 10 km exclusion zone 36 hours after the accident, initially evacuating around 49,000 people. This was later expanded to 30 km, resulting in the evacuation of approximately 68,000 more people. The government did not publicly acknowledge the disaster until two days after the explosion, when elevated radiation levels were detected in Sweden. Following the explosion, which killed two engineers and severely burned two others, an emergency operation began to put out the fires and stabilize the reactor. Of the 237 workers hospitalized, 134 showed symptoms of acute radiation syndrome (ARS); 28 of them died within three months. Over the next decade, 14 more workers (nine of whom had ARS) died of various causes mostly unrelated to radiation exposure. It is the only instance in commercial nuclear power history where radiation-related fatalities occurred. As of 2005, 6000 cases of childhood thyroid cancer occurred within the affected populations (15 of them fatal), "a large fraction" being attributed to the disaster. Long-term death estimates range from up to 4,000 in Ukraine, Belarus, and Russia (per the United Nations) to 16,000 in total across Europe. Pripyat was abandoned and replaced by the purpose-built city of Slavutych. The Chernobyl Nuclear Power Plant sarcophagus, completed in December 1986, reduced the spread of radioactive contamination and provided radiological protection for the crews of the undamaged reactors. In 2016–2018, the Chernobyl New Safe Confinement was constructed around the old sarcophagus to enable the removal of the reactor debris, with clean-up scheduled for completion by 2065.

The Manhattan Project was a research and development program undertaken during World War II to produce the first nuclear weapons. It was led by the United States in collaboration with the United Kingdom and Canada. The Manhattan Project employed nearly 130,000 people at its peak and cost nearly US$2 billion (equivalent to about $28 billion in 2024). From 1942 to 1946, the project was directed by Major General Leslie Groves of the U.S. Army Corps of Engineers. Nuclear physicist J. Robert Oppenheimer was the director of the Los Alamos Laboratory that designed the bombs. The Army program was designated the Manhattan District, as its first headquarters were in Manhattan; the name gradually superseded the official codename, Development of Substitute Materials, for the entire project. The project absorbed its earlier British counterpart, Tube Alloys. Over 80 percent of project cost was for building and operating the fissile material production plants. The project proposed both highly enriched uranium and plutonium as fuel for nuclear weapons. Enriched uranium was produced at the Clinton Engineer Works in Tennessee. Plutonium was produced in the world's first industrial-scale nuclear reactors at the Hanford Engineer Works in Washington. These sites were supported by dozens of other facilities across Canada, the US and the UK. The weapons designs were produced at the Los Alamos Laboratory in New Mexico, and resulted in two weapon designs used during the war: Little Boy (enriched uranium gun-type) and Fat Man (plutonium implosion). The Fat Man design was initially a low priority fallback option, as it was complex and required explosive lenses, but in 1944 it was confirmed that plutonium from Hanford was not suitable for a gun-type bomb because of the high proportion of plutonium-240. The first nuclear device ever detonated was an implosion-type bomb during the Trinity test, conducted at White Sands Proving Ground in New Mexico on 16 July 1945. The project was responsible for developing the specific means of delivering the weapons onto military targets, and for the use of the Little Boy and Fat Man bombs in the atomic bombings of Hiroshima and Nagasaki in August 1945. The project was also charged with gathering intelligence on the German nuclear weapon project. Through Operation Alsos, Manhattan Project personnel served in Europe, sometimes behind enemy lines, where they gathered nuclear materials and documents and rounded up German scientists. Despite the Manhattan Project's own emphasis on security, Soviet atomic spies penetrated the program. In the immediate postwar years, the Manhattan Project conducted weapons testing at Bikini Atoll as part of Operation Crossroads, developed new weapons, promoted the development of the network of national laboratories, supported medical research into radiology, and laid the foundations for the nuclear navy. It maintained control over American atomic weapons research and production until the formation of the United States Atomic Energy Commission (AEC) in January 1947.

The Battle of Waterloo was fought on Sunday 18 June 1815, near Waterloo (then in the United Kingdom of the Netherlands, now in Belgium), being the last engagement with Napoleon I. The French Imperial Army under the command of Napoleon I was defeated by two armies of the Seventh Coalition. One was a British-led force with units from the United Kingdom, the Netherlands, Hanover, Brunswick, and Nassau, under the command of field marshal Arthur Wellesley, Duke of Wellington. The other comprised three corps of the Prussian army under Field Marshal Blücher. The battle was known contemporaneously as the Battle of Mont Saint-Jean in France (after the hamlet of Mont-Saint-Jean) and La Belle Alliance in Prussia ("the Beautiful Alliance"; after the inn of La Belle Alliance). Upon Napoleon's return to power in March 1815, the beginning of the Hundred Days, many states that had previously opposed him formed the Seventh Coalition to oppose him again, and hurriedly mobilised their armies. Wellington's and Blücher's armies were cantoned close to the northeastern border of France. Napoleon planned to attack them separately, before they could link up and invade France with other members of the coalition. On 16 June, Napoleon successfully attacked the bulk of the Prussian Army at the Battle of Ligny with his main force, while a small portion of the French Imperial Army contested the Battle of Quatre Bras to prevent the Anglo-allied army from reinforcing the Prussians. The Anglo-allied army held their ground at Quatre Bras but were prevented from reinforcing the Prussians, and on the 17th, the Prussians withdrew from Ligny in good order, while Wellington then withdrew in parallel with the Prussians northward to Waterloo on 17 June. Napoleon sent a third of his forces to pursue the Prussians, which resulted in the separate Battle of Wavre with the Prussian rear-guard on 18–19 June and prevented that French force from participating at Waterloo. Upon learning that the Prussian Army was able to support him, Wellington decided to offer battle on the Mont-Saint-Jean escarpment across the Brussels Road, near the village of Waterloo. Here he withstood repeated attacks by the French throughout the afternoon of 18 June, and was eventually aided by the progressively arriving 50,000 Prussians who attacked the French flank and inflicted heavy casualties. In the evening, Napoleon assaulted the Anglo-allied line with his last reserves, the senior infantry battalions of the Imperial Guard. With the Prussians breaking through on the French right flank, the Anglo-allied army repulsed the Imperial Guard, and the French army was routed. Waterloo was the decisive engagement of the Waterloo campaign and Napoleon's last. It was the second-bloodiest single day battle of the Napoleonic Wars, after Borodino. According to Wellington, the battle was "the nearest-run thing you ever saw in your life". Napoleon abdicated four days later, and coalition forces entered Paris on 7 July. The defeat at Waterloo marked the end of Napoleon's Hundred Days return from exile. It precipitated Napoleon's second and definitive abdication as Emperor of the French, and ended the First French Empire. It set a historical milestone between serial European wars and decades of relative peace, often referred to as the Pax Britannica. In popular culture, the phrase "meeting one's Waterloo" has become an expression for experiencing a catastrophic reversal or undoing.

The Great Fire of London was a major conflagration that occurred in central London from Sunday 2 September to Wednesday 5 September 1666, gutting the medieval City of London inside the old Roman city wall, while also extending past the wall to the west. The death toll is generally thought to have been relatively small, although some historians have challenged this belief. The fire started in a bakery in Pudding Lane shortly after midnight on Sunday 2 September, and spread rapidly. The use of the major firefighting technique of the time, the creation of firebreaks by means of removing structures in the fire's path, was critically delayed by the hesitation of the Lord Mayor, Sir Thomas Bloodworth. By the time large-scale demolitions were ordered on Sunday night, the wind had already fanned the bakery fire into a firestorm which defeated such measures. The fire pushed north on Monday into the heart of the City. Order in the streets broke down as rumours arose of suspicious foreigners setting fires. The fears of the homeless focused on the French and Dutch, England's enemies in the ongoing Second Anglo-Dutch War; these substantial immigrant groups became victims of street violence. On Tuesday, the fire spread over nearly the whole city, destroying St Paul's Cathedral and leaping the River Fleet to threaten Charles II's court at Whitehall Palace. Coordinated firefighting efforts were simultaneously getting underway. The battle to put out the fire is considered to have been won by two key factors: the strong east wind dropped, and the Tower of London garrison used gunpowder to create effective firebreaks, halting further spread eastward. The social and economic problems created by the disaster were overwhelming. Flight from London and settlement elsewhere were strongly encouraged by Charles II, who feared a London rebellion amongst the dispossessed refugees. Various schemes for rebuilding the city were proposed, some of them radical. After the fire, London was reconstructed on essentially the same medieval street plan, which still exists today.

The Cuban Missile Crisis, also known as the October Crisis in Cuba (Spanish: Crisis de Octubre), or the Caribbean Crisis (Russian: Карибский кризис, romanized: Karibskiy krizis), was a 13-day confrontation between the governments of the United States and the Soviet Union, when American deployments of nuclear missiles in the United Kingdom, Italy and Turkey were matched by Soviet deployments of nuclear missiles in Cuba. The crisis lasted from 16 to 28 October 1962. The confrontation is widely considered the closest the Cold War came to escalating into full-scale nuclear war. In 1959, the US government deployed Thor nuclear missiles in England, an initiative known as Project Emily. In 1961 the US put Jupiter nuclear missiles in Italy and Turkey. All were within range of Moscow. The US had also trained a paramilitary force of Cuban expatriates, for a CIA-led attempt to invade Cuba and overthrow its government in April 1961. Starting in November of that year, the US government engaged in a violent campaign of terrorism and sabotage in Cuba, referred to as the Cuban Project, which continued throughout the first half of the 1960s. The Soviet administration was concerned about a Cuban drift towards China, with which the Soviets had an increasingly strained relationship. In response to these factors the Soviet and Cuban governments agreed, at a meeting between leaders Nikita Khrushchev and Fidel Castro in July 1962, to place nuclear missiles on Cuba to deter a future US invasion. Construction of launch facilities started shortly thereafter. A U-2 spy plane captured photographic evidence of medium- and long-range launch facilities in October. US president John F. Kennedy convened a meeting of the National Security Council and other key advisers, forming the Executive Committee of the National Security Council (EXCOMM). Kennedy was advised to carry out an air strike on Cuban soil in order to compromise Soviet missile supplies, followed by an invasion of the Cuban mainland. He chose a less aggressive course in order to avoid a declaration of war. On 22 October, Kennedy ordered a naval blockade to prevent further missiles from reaching Cuba. He referred to the blockade as a "quarantine", not as a blockade, so the US could avoid the formal implications of a state of war. An agreement was eventually reached between Kennedy and Khrushchev. The Soviets would dismantle their offensive weapons in Cuba, subject to United Nations verification, in exchange for a US public declaration and agreement not to invade Cuba again. The United States secretly agreed to dismantle all of the offensive weapons it had deployed to Turkey. All Thors in the UK were disbanded by August 1963. While the Soviets dismantled their missiles, some Soviet bombers remained in Cuba, and the United States kept the naval quarantine in place until 20 November 1962. The blockade was formally ended on 20 November after all offensive missiles and bombers had been withdrawn from Cuba. The evident necessity of a quick and direct communication line between the two powers resulted in the Moscow–Washington hotline. A series of agreements later reduced US–Soviet tensions for several years. The compromise embarrassed Khrushchev and the Soviet Union because the withdrawal of US missiles from Italy and Turkey was a secret deal between Kennedy and Khrushchev, and the Soviets were seen as retreating from a situation that they had started. Khrushchev's fall from power two years later was in part because of the Soviet Politburo's embarrassment at both Khrushchev's eventual concessions to the US and his ineptitude in precipitating the crisis. According to the Soviet ambassador to the United States, Anatoly Dobrynin, the top Soviet leadership took the Cuban outcome as "a blow to its prestige bordering on humiliation".

The American Revolution (1765–1789) was a political movement in the Thirteen Colonies of Great Britain. The movement began as a rebellion and evolved into a revolution resulting in the sovereign United States. These changes were the outcome of the associated American Revolutionary War. The Second Continental Congress, as the provisional government, established the Continental Army and appointed George Washington as its commander-in-chief in 1775. The following year, the Congress passed the Lee Resolution on July 2nd, then unanimously adopted the Declaration of Independence on the Fourth of July. Throughout most of the war, the outcome appeared uncertain. However, in 1781, a decisive victory by Washington and the Continental Army in the Siege of Yorktown led King George III and the Fox–North coalition in government to negotiate the cessation of colonial rule and the acknowledgment of American sovereignty, formalized in the Treaty of Paris in 1783. The Constitution took effect in 1789 and the Bill of Rights was ratified in 1791. Discontent with colonial rule began shortly after the French and Indian War ended in 1763. Even though the colonies had fought in and supported the war directly with troops, funds and material, the British Parliament imposed new taxes to ostensibly compensate for wartime costs, and transferred control of the colonies' western lands to British officials in Montreal. Representatives from several colonies convened in New York City for the Stamp Act Congress in 1765; its "Declaration of Rights and Grievances" argued that this taxation without representation in Parliament and other policies violated their rights as Englishmen. In 1767, though the Stamp Act was repealed, tensions flared again following Parliament's passage of the Townshend Acts. In an effort to quell the mounting rebellion, King George III deployed British troops to the colony of Massachusetts, where they killed antagonists in the 1770 Boston Massacre. In December 1773, the local faction of the colonies-wide Sons of Liberty underground orchestrated the Boston Tea Party, during which they dumped chests of taxed tea owned by the British East India Company into Boston Harbor. Parliament responded by enacting a series of punitive laws, intended to end self-government in Massachusetts, but instead increased American support for the revolutionary cause. In 1774, twelve of the Thirteen Colonies sent delegates to the First Continental Congress; the Province of Georgia joined in 1775. The First Continental Congress began coordinating Patriot resistance through underground networks of committees largely built on the foundations of the Sons of Liberty network. In August 1775, King George III proclaimed Massachusetts to be in a state of rebellion. The British attempted to disarm the colonists, resulting in the Battles of Lexington and Concord, sparking the Revolutionary War. The Second Continental Congress convened in May of 1775 and created the Continental Army, which then surrounded Boston, forcing the British to withdraw by sea in March 1776 and leaving Patriots in control in every colony. In May 1776, Congress voted to suppress all forms of Crown authority, to be replaced by locally created authority, and each colony created a state constitution. On July 2, the Congress passed the Lee Resolution, affirming their support for national independence. On July 4, 1776 they unanimously adopted the Declaration of Independence, famously proclaiming that "all men are created equal", having then evolved from a rebellion over British rights to a revolution based on universal rights. The Congress soon after began deliberating the Articles of Confederation, an effort to establish a multi-state self-governing coordinating body capable of negotiating international treaties and prosecuting the war. The Revolutionary War continued for another five years during which France ultimately entered, supporting the revolutionary cause. On September 28, 1781, Washington commanded the Continental Army's capture of a British army under General Cornwallis at the Siege of Yorktown, leading to the collapse of King George's control of Parliament. Consensus in Parliament soon shifted to the war ending on American terms. On September 3, 1783, the British signed the Treaty of Paris, recognizing the sovereign independence of the United States, and ceding to the new nation nearly all the territory east of the Mississippi River and south of the Great Lakes. The United States became the first nation to establish a federal republic with a written constitution based on the principles of universal natural rights, consent of the governed, and equality under the law, albeit with significant democratic limitations compared to later evolution of the concept.

The Crusades were a series of military campaigns launched by the papacy between 1095 and 1291 against Muslim rulers for the recovery and defence of the Holy Land, as part of a wider crusading movement. The First Crusade was proclaimed by Pope Urban II at the Council of Clermont in November 1095—a call to arms for Christians to reconquer Jerusalem from the Muslims, with promises of spiritual reward. By this time, the papacy's position as head of the Catholic Church had strengthened, and earlier conflicts with secular rulers and wars on the frontiers of Western Christendom had prepared it for the direction of armed force in religious causes. The successes of the First Crusade led to the establishment of four Crusader states in the Levant, where their defence required further expeditions from Catholic Europe. The organisation of such large-scale campaigns demanded complex religious, social, and economic institutions, including crusade indulgences, military orders, and the taxation of clerical income. Over time, the crusading movement expanded to include campaigns against pagans, Christian dissidents, and other enemies of the papacy, promoted with similar spiritual rewards and continuing into the 18th century. The Crusade of 1101, the earliest papally sanctioned expedition inspired by the First Crusade, ended in disastrous defeats. For several decades thereafter, only smaller expeditions reached the Holy Land, yet their role in consolidating and expanding the Crusader states was pivotal. The fall of Edessa, the capital of the first Crusader state, prompted the Second Crusade, which failed in 1148. Its failure reduced support for crusading across Latin Christendom, leaving the Crusader states unable to resist Saladin's expansion. Having united Egypt and Muslim Syria under his rule, Saladin destroyed their combined armies at the Battle of Hattin in 1187. The Crusader states survived largely owing to the Third Crusade, a major campaign against Saladin, though Jerusalem remained under Muslim control. Initially directed against Egypt, the Fourth Crusade was diverted to the Byzantine Empire, culminating in the Sack of Constantinople and the establishment of the Latin Empire in 1204. The Fifth Crusade again targeted Egypt but failed to conquer it in 1219–21. By this period, crusade indulgences could also be obtained through other campaigns—such as the Iberian, Albigensian, and Northern Crusades—thereby diminishing enthusiasm for expeditions in the eastern Mediterranean. Jerusalem was regained through negotiation during the Sixth Crusade in 1229, and in 1239–41 the Barons' Crusade restored much of the territory the Crusader states had lost. However, the Sack of Jerusalem by Muslim freebooters soon ended Crusader rule in the Holy City. Louis IX of France launched two major campaigns—the Seventh Crusade against Egypt in 1248–51 and the Eighth Crusade against Tunis in 1270—both of which ended in failure. In place of the large-scale passagium generale, the smaller passagium particulare became the predominant form of crusading campaigns in the late 13th century. The Crusader states, however, were unable to withstand the advance of the Mamluks. Having reunited Egypt and Muslim Syria by 1260, they went on to attack the Crusader states, capturing the Crusaders' last mainland strongholds in 1291. Although plans for the reconquest of the Holy Land continued to be made in the following decades, only the Alexandrian Crusade briefly revived crusading activity in the region in 1365.

The Silk Road was a network of Asian trade routes active from the second century BCE until the mid-15th century. Spanning over 6,400 km (4,000 mi) on land, it played a central role in facilitating economic, cultural, political, and religious interactions between the Eastern and Western worlds. The name "Silk Road" was coined in the late 19th century, but some 20th- and 21st-century historians instead prefer the term Silk Routes, on the grounds that it more accurately describes the intricate web of land and sea routes connecting Central, East, South, Southeast, and West Asia as well as East Africa and Southern Europe. In fact, some scholars criticise or even dismiss the idea of silk roads and call for a new definition or alternate term. According to them, the literature using this term has "privileged the sedentary and literate empires at either end of Eurasia" thereby ignoring the contributions of steppe nomads. In addition, the classic definition sidelines prominent civilisations such as India and Iran. The Silk Road derives its name from the highly lucrative trade of silk textiles that were primarily produced in China. The network began with the expansion of the Han dynasty (202 BCE – 220 CE) into Central Asia around 114 BCE, through the missions and explorations of the Chinese imperial envoy Zhang Qian, which brought the region under unified control. The Chinese took great interest in the security of their trade products, and extended the Great Wall of China to ensure the protection of the trade route. The Parthian Empire provided a vital bridge connecting the network to the Mediterranean. Meanwhile, the rise of the Roman Empire in the west further established the western terminus of the interconnected trade system. By the first century CE, Chinese silk was widely sought after in Rome, Egypt, and Greece. Other lucrative commodities from the East included tea, dyes, perfumes, and porcelain; among Western exports were horses, camels, honey, wine, and gold. Aside from generating substantial wealth for emerging mercantile classes, the proliferation of goods such as paper and gunpowder greatly affected the trajectory of political history in several theatres in Eurasia and beyond. The Silk Road was utilized over a period that saw immense political variation across the continent, exemplified by major events such as the Black Death and the Mongol conquests. The network was highly decentralized, and security was sparse: travelers faced constant threats of banditry and nomadic raiders, and long expanses of inhospitable terrain. Few individuals traveled the entire length of the Silk Road, instead relying on a succession of middlemen based at various stopping points along the way. In addition to goods, the network facilitated an unprecedented exchange of religious (especially Buddhist), philosophical, and scientific thought, much of which was syncretised by societies along the way. Likewise, a wide variety of people used the routes. Diseases such as plague also spread along the Silk Road, possibly contributing to the Black Death. From 1453 onwards, the Ottoman Empire began competing with other gunpowder empires for greater control over the overland routes, which prompted European polities to seek alternatives while themselves gaining leverage over their trade partners. This marked the beginning of the Age of Discovery, European colonialism, and the further intensification of globalization. In the 21st century, the name "New Silk Road" is used to describe several large infrastructure projects along many of the historic trade routes; among the best known include the Eurasian Land Bridge and the Chinese Belt and Road Initiative (BRI). UNESCO designated the Chang'an-Tianshan corridor of the Silk Road as a World Heritage Site in 2014, and the Zarafshan-Karakum Corridor in 2023. The Fergana-Syrdarya Corridor, the Indian and Iranian portions, and the remaining sites in China remain on the tentative lists. Despite the popular imagination, Silk Road was never a singular east-west trade route that linked China to the Mediterranean, nor was there unrestricted trade before the Mongol Empire. It was a network of routes. Even Marco Polo, often linked to the Silk Road, never used the term despite traveling during a time of Mongol-enabled ease of movement.

The Ottoman Empire, historically also known as the Turkish Empire, was a state that spanned much of Southeast Europe, West Asia, and North Africa from the 14th century to the early 20th century, centred in modern-day Turkey. It also controlled parts of southeastern Central Europe between the early 16th and early 18th centuries. The empire emerged from a beylik, or principality, founded in northwestern Anatolia in c. 1299 by the Turkoman tribal leader Osman I. His successors conquered much of Anatolia and expanded into the Balkans by the mid-14th century, transforming their petty kingdom into a transcontinental empire. The Ottomans ended the Byzantine Empire with the conquest of Constantinople in 1453 by Mehmed II. Further conquests by Selim I led the Sultans to adopt the title of caliph. With its capital at Constantinople and control over a significant portion of the Mediterranean Basin, the Ottoman Empire was at the centre of interactions between the Middle East and Europe for six centuries. Ruling over so many peoples, the empire granted varying levels of autonomy to its many confessional communities, or millets, to manage their own affairs per Islamic law. During the reign of Suleiman the Magnificent in the 16th century, the Ottoman Empire became a global power. Modern academic consensus posits that the empire began to decline after defeat in the Second Siege of Vienna in 1683, but continued to maintain a flexible and strong economy, society and military into much of the 18th century. The Ottoman Empire fell behind technologically from the rest of Europe by the late 18th century as imperial authority fragmented. Further defeats from Austria and Russia culminated in the loss of territory, and with rising nationalism after the French Revolution, a number of new states emerged in the Balkans. Following Mahmud II's reign and the Tanzimat reforms over the course of the 19th century, the Ottoman state became more powerful and organised internally as a new Ottoman identity took hold. In the 1876 revolution, the Ottoman Empire attempted constitutional monarchy, before reverting to an absolute monarchy under Abdul Hamid II, following the Great Eastern Crisis. Over the course of the late 19th century, Ottoman intellectuals known as Young Turks sought to liberalise and rationalise society and politics along Western lines, culminating in the Young Turk Revolution of 1908 led by the Committee of Union and Progress (CUP), which reestablished a constitutional monarchy. However, following the disastrous Balkan Wars, the CUP became increasingly radicalised and Turkish nationalist, leading a coup d'état in 1913 that established a dictatorship. In the 19th and early 20th centuries, persecution of Muslims during the Ottoman contraction and in the Russian Empire resulted in large-scale loss of life and mass migration into modern-day Turkey from the Balkans, Caucasus, and Crimea. The CUP joined World War I on the side of the Central Powers. It struggled with internal dissent, especially the Arab Revolt, and engaged in genocide against Armenians, Assyrians, and Greeks. In the aftermath of World War I, the victorious Allied Powers occupied and partitioned the Ottoman Empire, which lost its southern territories to the United Kingdom and France. The successful Turkish War of Independence, led by Mustafa Kemal Atatürk against the occupying Allies, led to the end of the Ottoman sultanate in 1922.

Enlightenment or enlighten may refer to:

Feudalism, also known as the feudal system, was a combination of various customs and systems that flourished in medieval Europe from the 9th to 15th centuries. Broadly defined, it was a way of structuring society around relationships derived from the holding of land in exchange for service or labour. The classic definition, by François Louis Ganshof (1944), describes a set of reciprocal legal and military obligations of the warrior nobility and revolved around the key concepts of lords, vassals, and fiefs. A broader definition, as described by Marc Bloch (1939), includes not only the obligations of the warrior nobility but the obligations of all three estates of the realm: the nobility, the clergy, and the peasantry, all of whom were bound by a system of manorialism; this is sometimes referred to as a "feudal society". Although it is derived from the Latin word feodum or feudum (fief), which was used during the medieval period, the term feudalism and the system it describes were not conceived of as a formal political system by the people who lived during the Middle Ages. Since the publication of Elizabeth A. R. Brown's "The Tyranny of a Construct" (1974) and Susan Reynolds's Fiefs and Vassals (1994), there has been ongoing inconclusive discussion among medieval historians as to whether feudalism is a useful construct for understanding medieval society. Some scholars have also applied the label outside of Europe, including feudal Japan, medieval Ethiopia, Spring and Autumn period China, ancient Egypt, the Parthian Empire, India until the Mughal dynasty and the Antebellum South and Jim Crow laws in the American South.

The Aztec Empire, also known as the Triple Alliance (Classical Nahuatl: Ēxcān Tlahtōlōyān, [ˈjéːʃkaːn̥ t͡ɬaʔtoːˈlóːjaːn̥]) or historiographically as the Tenochca Empire but most accurately known as the Mexica, was an alliance of three Nahua city-states: Mexico-Tenochtitlan, Tetzcoco, and Tlacopan. These three city-states ruled that area in and around the Valley of Mexico from 1428 until the combined forces of the Spanish conquistadores and their native allies who ruled under Hernán Cortés defeated them in 1521. Its people and civil society are historiographically referred to as the Aztecs or the Culhua-Mexica. The alliance was formed from the victorious factions of a civil war fought between the city of Azcapotzalco and its former tributary provinces. Despite the initial conception of the empire as an alliance of three self-governed city-states, the capital Tenochtitlan became dominant militarily. By the time the Spanish arrived in 1519, the lands of the alliance were effectively ruled from Tenochtitlan, while other partners of the alliance had taken subsidiary roles. The alliance waged wars of conquest and expanded after its formation. The alliance controlled most of central Mexico at its height, as well as some more distant territories within Mesoamerica, such as the Xoconochco province, an Aztec exclave near the present-day Guatemalan border. Aztec rule has been described by scholars as hegemonic or indirect. The Aztecs left rulers of conquered cities in power so long as they agreed to pay semi-annual tribute to the alliance, as well as supply military forces when needed for the Aztec war efforts. In return, the imperial authority offered protection and political stability and facilitated an integrated economic network of diverse lands and peoples who had significant local autonomy. Aztec religion was a monistic pantheism in which the Nahua concept of teotl was construed as the supreme god Ometeotl, as well as a diverse pantheon of lesser gods and manifestations of nature. The popular religion tended to embrace the mythological and polytheistic aspects, and the empire's state religion sponsored both the monism of the upper classes and the popular heterodoxies. The empire even officially recognized the largest cults such that the deity was represented in the central temple precinct of the capital Tenochtitlan. The imperial cult was specifically that of the distinctive warlike patron god of the Mexica Huītzilōpōchtli. Peoples were allowed to retain and freely continue their own religious traditions in conquered provinces so long as they added the imperial god Huītzilōpōchtli to their local pantheons.

The fall of the Western Roman Empire, also called the fall of the Roman Empire or the fall of Rome, was the loss of central political control in the Western Roman Empire, a process in which the Empire failed to enforce its rule, and its vast territory was divided among several successor polities. The Roman Empire lost the strengths that had allowed it to exercise effective control over its Western provinces; modern historians posit factors including the effectiveness and numbers of the army, the health and numbers of the Roman population, the strength of the economy, the competence of the emperors, the internal struggles for power, the religious changes of the period, and the efficiency of the civil administration. Increasing pressure from invading peoples outside Roman culture also contributed greatly to the collapse. Climatic changes and both endemic and epidemic diseases drove many of these immediate factors. The reasons for the collapse are major subjects of the historiography of the ancient world and they inform much modern discourse on state failure. In 376, a large migration of Goths and other non-Roman people, fleeing from the Huns, entered the Empire. Roman forces were unable to exterminate, expel or subjugate them (as was their normal practice). In 395, after winning two destructive civil wars, Theodosius I died. He left a collapsing field army, and the Empire divided between the warring ministers of his two incapable sons. Goths and other non-Romans became a force that could challenge either part of the Empire. Further barbarian groups crossed the Rhine and other frontiers. The armed forces of the Western Empire became few and ineffective, and despite brief recoveries under able leaders, central rule was never again effectively consolidated. By 476, the position of Western Roman Emperor wielded negligible military, political, or financial power, and had no effective control over the scattered Western domains that could still be described as Roman. Barbarian kingdoms had established their own power in much of the area of the Western Empire. In 476, the Germanic barbarian king Odoacer deposed the last emperor of the Western Roman Empire in Italy, Romulus Augustulus, and the Senate sent the imperial insignia to the Eastern Roman emperor Zeno. While its legitimacy lasted for centuries longer and its cultural influence remains, the Western Empire never had the strength to rise again. The Eastern Roman, or Byzantine, Empire, survived and remained for centuries an effective power of the Eastern Mediterranean, although it lessened in strength. While the loss of political unity and military control is universally acknowledged, the fall of Rome is not the only unifying concept for these events; the period described as late antiquity emphasizes the cultural continuities throughout and beyond the political collapse.

The Byzantine Empire, also known as the Eastern Roman Empire, was the continuation of the Roman Empire centred on Constantinople during late antiquity and the Middle Ages. Having survived the events that caused the fall of the Western Roman Empire in the 5th century AD, it endured until the fall of Constantinople to the Ottoman Empire in 1453. The term 'Byzantine Empire' was coined only after its demise; its citizens used the term 'Roman Empire' and called themselves 'Romans'. During the early centuries of the Roman Empire, the western provinces were Latinised, but the eastern parts kept their Hellenistic culture. Constantine I (r. 324–337) legalised Christianity and moved the capital to Constantinople. Theodosius I (r. 379–395) made Christianity the state religion and Greek gradually replaced Latin for official use. The empire adopted a defensive strategy and, throughout its remaining history, experienced recurring cycles of decline and recovery. The Byzantine Empire reached its greatest extent under the reign of Justinian I (r. 527–565), who briefly reconquered much of Italy and the western Mediterranean coast. A plague began around 541, and a prolonged warfare with Persia placed fiscal and military strain on the empire, contributing to political and strategic challenges in the decades that followed. In the 630s and 640s the Arab conquests defeated Byzantine field armies in Syria and Egypt, resulting in the permanent loss of those provinces to the Rashidun Caliphate. In 698, Africa was lost to the Umayyad Caliphate, but the empire stabilised under the Isaurian dynasty. It expanded once more under the Macedonian dynasty, experiencing a two-century-long renaissance. Thereafter, periods of civil war and Seljuk incursion resulted in the loss of most of Asia Minor. The empire recovered during the Komnenian restoration, and Constantinople remained the largest and wealthiest city in Europe until the 13th century, when it was overtaken by Paris. The empire was severely fragmented in 1204, following the sack of Constantinople during the Fourth Crusade; its former territories were then divided into competing Greek rump states and Latin realms. Despite the eventual recovery of Constantinople in 1261, the reconstituted empire wielded only regional power during its final two centuries. Its remaining territories were progressively annexed by the Ottomans in a series of wars fought in the 14th and 15th centuries. The fall of Constantinople to the Ottomans in 1453 brought the empire to an end.

The American Civil War (April 12, 1861 – May 26, 1865; also known by other names) was a civil war in the United States between the Union ("the North") and the Confederacy ("the South"), which was formed in 1861 by states that had seceded from the Union to preserve slavery in the United States, which they saw as threatened because of the election of Abraham Lincoln and the growing abolitionist movement in the North. The war ended with Union victory, the dissolution of the Confederacy and the abolition of slavery, freeing four million African Americans. Decades of controversy over slavery came to a head when Abraham Lincoln, a Republican who opposed slavery's expansion, won the 1860 presidential election. Seven Southern slave states responded to Lincoln's victory by seceding from the United States and forming the Confederacy. The Confederacy seized US forts and other federal assets in the South. The war began on April 12, 1861, when the Confederacy bombarded Fort Sumter in South Carolina. A wave of enthusiasm for war swept over the North and South, as military recruitment soared. Four more Southern states seceded after the war began and, led by its president, Jefferson Davis, the Confederacy comprised eleven states, containing a third of the US population. Four years of intense combat, mostly in the South, ensued. During 1861–1862 in the western theater, the Union made permanent gains, though in the eastern theater the conflict was inconclusive. The abolition of slavery became a Union war goal on January 1, 1863, when Lincoln issued the Emancipation Proclamation, which declared all slaves in rebel states to be free, applying to more than 3.5 million of the 4 million enslaved people in the country. To the west, the Union first destroyed the Confederacy's river navy by the summer of 1862, then much of its western armies, and seized New Orleans. The successful 1863 Union siege of Vicksburg split the Confederacy in two at the Mississippi River, while Confederate general Robert E. Lee's incursion north failed at the Battle of Gettysburg. General Ulysses S. Grant's western successes led Lincoln to promote him to command of all Union armies in 1864. Inflicting an ever-tightening naval blockade of Confederate ports, the Union marshaled resources and manpower to attack the Confederacy from all directions. This led to the fall of Atlanta in 1864 to Union general William Tecumseh Sherman, followed by his March to the Sea, which culminated in his taking Savannah. The last significant battles raged around the ten-month Siege of Petersburg, gateway to the Confederate capital of Richmond. The Confederates abandoned Richmond, and on April 9, 1865, Lee surrendered to Grant following the Battle of Appomattox Court House, setting in motion the end of the war. Lincoln lived to see this victory but was shot by an assassin on April 14, dying the next day. By the end of the war, much of the South's infrastructure had been destroyed. The Confederacy collapsed, the Thirteenth Amendment abolished slavery, and four million enslaved black people were freed. The war-torn nation then entered the Reconstruction era in an attempt to rebuild the country, bring the former Confederate states back into the United States, and grant civil rights to freed slaves. The war is one of the most extensively studied and written about episodes in the history of the United States. It remains the subject of cultural and historiographical debate. Of continuing interest is the myth of the Lost Cause of the Confederacy. The war was among the first to use industrial warfare. Railroads, the electrical telegraph, steamships, the ironclad warship, and mass-produced weapons were widely used. The war left an estimated 700,000 soldiers dead, along with an undetermined number of civilian deaths, making it the deadliest in American history. The technology and brutality of the Civil War foreshadowed the coming world wars.

In modern historiography, ancient Rome is the Roman civilisation from the founding of the Italian city of Rome in the 8th century BC to the collapse of the Western Roman Empire in the 5th century AD. It encompasses the Roman Kingdom (753–509 BC), the Roman Republic (509‍–‍27 BC), and the Roman Empire (27 BC – 476 AD) until the fall of the western empire. Ancient Rome began as an Italic settlement, traditionally dated to 753 BC, beside the River Tiber in the Italian Peninsula. The settlement grew into the city and polity of Rome, and came to control its neighbours through a combination of treaties and military strength. It eventually controlled the Italian Peninsula, assimilating the Greek culture of southern Italy (Magna Graecia) and the Etruscan culture, and then became the dominant power in the Mediterranean region and parts of Europe. At its height it controlled the North African coast, Egypt, Southern Europe, and most of Western Europe, the Balkans, Crimea, and much of the Middle East, including Anatolia, the Levant, and parts of Mesopotamia and Arabia. That empire was among the largest empires in the ancient world, covering around 5 million square kilometres (1.9 million square miles) in AD 117, with an estimated 50 to 90 million inhabitants, roughly 20% of the world's population at the time. The Roman state evolved from an elective monarchy to a classical republic and then to an increasingly autocratic military dictatorship during the Empire. Ancient Rome is often grouped into classical antiquity together with ancient Greece, and their similar cultures and societies are known as the Greco-Roman world. Ancient Roman civilisation has contributed to modern language, religion, society, technology, law, politics, government, warfare, art, literature, architecture, and engineering. Rome professionalised and expanded its military and created a system of government called res publica, the inspiration for modern republics such as the United States and France. It achieved impressive technological and architectural feats, such as the empire-wide construction of aqueducts and roads, as well as more grandiose monuments and facilities.

The Black Death was a plague pandemic that occurred in Europe from 1346 to 1353. It was one of the most fatal pandemics in human history; as many as 50 million people died, perhaps 50% of Europe's 14th-century population. The disease is caused by the bacterium Yersinia pestis and is spread by fleas and through the air. One of the most significant events in European history, the Black Death had far-reaching population, economic, and cultural effects. It was the beginning of the second plague pandemic. The plague created religious, social, and economic upheavals, with profound effects on the course of European history. The Black Death was the second great natural disaster to strike Europe during the Late Middle Ages (the first one being the Great Famine of 1315–1317) and is estimated to have killed 30% to 60% of the European population, as well as approximately 33% of the population of the Middle East. There were further outbreaks throughout the Late Middle Ages and, also due to other contributing factors (the crisis of the late Middle Ages), the European population did not regain its 14th century level until the 16th century. Outbreaks of the plague recurred around the world until the early 19th century.

The British Empire comprised the dominions, colonies, protectorates, mandates, and other territories ruled or administered by the United Kingdom and its predecessor states. It began with the overseas possessions and trading posts established by England in the late 16th and early 17th centuries, and colonisation attempts by Scotland during the 17th century. At its height in the 19th and early 20th centuries, it was the largest empire in history and, for a century, was the foremost global power. By 1913, the British Empire held sway over 412 million people, 23 per cent of the world population at the time, and by 1920, it covered 35.5 million km2 (13.7 million sq mi), 24 per cent of the Earth's total land area. As a result, its constitutional, legal, linguistic, and cultural legacy is widespread. It was described as "the empire on which the sun never sets", as the sun was always shining on at least one of its territories. During the Age of Discovery in the 15th and 16th centuries, Portugal and Spain pioneered European exploration of the world, and in the process established large overseas empires. Motivated by the great wealth these empires generated, England, France, and the Netherlands began to establish colonies and trade networks in the Americas and Asia. A series of wars in the 17th and 18th centuries with the Netherlands and France left Britain the dominant colonial power in North America. Britain became a major power in the Indian subcontinent after the East India Company's conquest of Mughal Bengal at the Battle of Plassey in 1757. The American Revolution resulted in Britain losing some of its oldest and most populous colonies in North America by 1783. While retaining control of British North America (now Canada) and territories in and near the Caribbean in the British West Indies, British colonial expansion turned towards Asia, Africa, and the Pacific. After the defeat of France in the Napoleonic Wars (1803–1815), Britain emerged as the principal naval and imperial power of the 19th century and expanded its imperial holdings. It pursued trade concessions in China and Japan, and territory in Southeast Asia. The Great Game and Scramble for Africa also ensued. The period of relative peace (1815–1914) during which the British Empire became the global hegemon was later described as Pax Britannica (Latin for "British Peace"). Alongside the formal control that Britain exerted over its colonies, its dominance of much of world trade, and of its oceans, meant that it effectively controlled the economies of, and readily enforced its interests in, many regions, such as Asia and Latin America. It also came to dominate the Middle East, shaping its modern borders alongside France. Increasing degrees of autonomy were recognised for its white settler colonies, some of which were formally reclassified as Dominions by the 1920s. By the start of the 20th century, Germany and the United States had begun to challenge Britain's economic lead. Military, economic and colonial tensions between Britain and Germany were major causes of the First World War, during which Britain relied heavily on its empire. The conflict placed enormous strain on its military, financial, and manpower resources. Although the empire achieved its largest territorial extent immediately after the First World War, Britain was no longer the world's preeminent industrial or military power. In the Second World War, Britain's colonies in East Asia and Southeast Asia were occupied by the Empire of Japan. Despite the final victory of Britain and its allies, the damage to British prestige and the British economy helped accelerate the decline of the empire. India, Britain's most valuable and populous possession, achieved independence in 1947 as part of a larger decolonisation movement, in which Britain granted independence to most territories of the empire. The Suez Crisis of 1956 confirmed Britain's decline as a global power, and the handover of Hong Kong to China on 1 July 1997 symbolised for many the end of the British Empire, though fourteen overseas territories that are remnants of the empire remain under British sovereignty. After independence, many former British colonies, along with most of the dominions, joined the Commonwealth of Nations, a free association of independent states. Fifteen of these, including the United Kingdom, retain the same person as monarch, currently King Charles III.

World War I, or the First World War (28 July 1914 – 11 November 1918), also known as the Great War, was a global conflict between two coalitions: the Allies (or Entente) and the Central Powers. Major areas of conflict included Europe and the Middle East, as well as parts of Africa and the Asia-Pacific. The war saw important developments in weaponry including tanks, aircraft, artillery, machine guns, and chemical weapons. One of the deadliest conflicts in history, it resulted in an estimated 15 to 22 million military and civilian casualties and genocide. The movement of large numbers of people was a major factor in the deadly Spanish flu pandemic. The causes of World War I included the rise of the German Empire and decline of the Ottoman Empire, which disturbed the long-standing balance of power in Europe, the exacerbation of imperial rivalries, and an arms race between the great powers. Growing tensions in the Balkans reached a breaking point on 28 June 1914 when Gavrilo Princip, a Bosnian Serb, assassinated Franz Ferdinand, the heir to the Austro-Hungarian throne. Austria-Hungary blamed Serbia, and declared war on 28 July. After Russia mobilised in Serbia's defence, Germany declared war on Russia and France, who had an alliance. The United Kingdom entered the war after Germany invaded Belgium, and the Ottoman Empire joined the Central Powers in November. Germany's strategy in 1914 was to quickly defeat France before transferring its forces to the east, but its advance was halted in September, and by the end of the year the Western Front consisted of a near-continuous line of trenches from the English Channel to Switzerland. The Eastern Front was more dynamic, but neither side gained a decisive advantage, despite costly offensives. Italy, Bulgaria, Romania, Greece, and others entered the war from 1915 onward. Major battles, including those at Verdun, the Somme, and Passchendaele, failed to break the stalemate on the Western Front. In April 1917, the United States joined the Allies after Germany resumed unrestricted submarine warfare against Atlantic shipping. Later that year, the Bolsheviks seized power in Russia in the October Revolution; Soviet Russia signed an armistice with the Central Powers in December, followed by a separate peace in March 1918. That month, Germany launched a spring offensive in the west, which despite initial successes left the German Army exhausted and demoralised. The Allied Hundred Days Offensive, beginning in August 1918, caused a collapse of the German front line. Following the Vardar Offensive, Bulgaria signed an armistice in late September. By early November, the Allies had signed armistices with the Ottomans and with Austria-Hungary, leaving Germany isolated. Facing a revolution at home, Kaiser Wilhelm II abdicated on 9 November, and the war ended with the Armistice of 11 November 1918. The Paris Peace Conference of 1919–1920 imposed settlements on the defeated powers. Under the Treaty of Versailles, Germany lost significant territories, was disarmed, and was required to pay large war reparations to the Allies. The dissolution of the Russian, German, Austro-Hungarian, and Ottoman empires led to new national boundaries and the creation of new independent states including Poland, Finland, the Baltic states, Czechoslovakia, and Yugoslavia. The League of Nations was established to maintain world peace, but its failure to manage instability during the interwar period contributed to the outbreak of World War II in 1939.

In the history of Europe, the Middle Ages or medieval period lasted approximately from the 5th to late 15th centuries, comparable with the post-classical period of global history. The medieval period is the middle epoch of the three traditional divisions of Western history: classical antiquity, the medieval period, and the modern period. The medieval period is itself subdivided into the Early, High, and Late Middle Ages. The Middle Ages began with the fall of the Western Roman Empire and transitioned into the Modern age during the Renaissance, the Reformation, the European wars of religion, and the Age of Discovery. Early Modernity is a single term adopted to describe this end-stage of the period, during which medieval Europe shades into the modern period. Population decline, counterurbanisation, the collapse of centralised authority, invasions, and mass migrations of tribes had begun in late antiquity (c. ~3rd–8th centuries) and lasted into the Early Middle Ages (c. 5th–10th centuries). The large-scale movements of the Migration Period, including various Germanic peoples, formed new kingdoms in what remained of the Western Roman Empire. In the 7th century, North Africa and the Middle East—once part of the Byzantine Empire—came under the rule of the Umayyad Caliphate, an Islamic empire, after conquest by Muhammad's successors. Although there were substantial changes in society and political structures, the break with classical antiquity was incomplete. The still-sizeable Byzantine Empire, Rome's direct continuation, survived in the Eastern Mediterranean and remained a major power. The empire's law code, the Corpus Juris Civilis or "Code of Justinian", was rediscovered in Northern Italy in the 11th century. In the West, most kingdoms incorporated the few extant Roman institutions. Monasteries were founded as campaigns to Christianise the remaining pagans across Europe continued. The Franks, under the Carolingian dynasty, briefly established the Carolingian Empire during the later 8th and early 9th centuries. It covered much of Western Europe but later succumbed to the pressures of internal civil wars combined with external invasions: Vikings from the north, Magyars from the east, and Saracens from the south. During the High Middle Ages, which began after 1000, the population of Europe increased significantly as technological and agricultural innovations allowed trade to flourish and the Medieval Warm Period climate change allowed crop yields to increase. Manorialism, the organisation of peasants into villages that owed rent and labour services to the nobles, and feudalism, the political structure whereby knights and lower-status nobles owed military service to their overlords in return for the right to rent from lands and manors, were two of the ways society was organised in the High Middle Ages. This period also saw the collapse of the unified Christian church with the East–West Schism of 1054. The Crusades, first preached in 1095, were military attempts by Western European Christians to regain control of the Holy Land from Muslims. Kings became the heads of centralised nation-states, reducing crime and violence but making the ideal of a unified Christendom more distant. Intellectual life was marked by scholasticism, a philosophy that emphasised joining faith to reason, and by the founding of universities. The theology of Thomas Aquinas, the paintings of Giotto, the poetry of Dante and Chaucer, the travels of Marco Polo, and the Gothic architecture of cathedrals such as Chartres are among the outstanding achievements toward the end of this period and into the Late Middle Ages. The Late Middle Ages was marked by difficulties and calamities, including famine, plague, and war, which significantly diminished the population of Europe; between 1347 and 1350, the Black Death killed about a third of Europeans. Controversy, heresy, and the Western Schism within the Catholic Church paralleled the interstate conflict, civil strife, and peasant revolts that occurred in the kingdoms. Cultural and technological developments transformed European society, concluding the Late Middle Ages and beginning the early modern period.

The Inca Empire, officially known as the Realm of the Four Parts (Quechua: Tawantinsuyu pronounced [taˈwantiŋ ˈsuju], lit. 'land of four parts'), was the largest empire in pre-Columbian America. The administrative, political, and military center of the empire was in the city of Cusco. The Inca civilisation rose from the Peruvian highlands sometime in the early 13th century. The Portuguese explorer Aleixo Garcia was the first European to reach the Inca Empire in 1524. Later, in 1532, the Spanish began the conquest of the Inca Empire, and by 1572 the last Inca state was fully conquered. From 1438 to 1533, the Incas incorporated a large portion of western South America, centered on the Andean Mountains, using conquest and peaceful assimilation, among other methods. At its largest, the empire joined modern-day Peru with what are now western Ecuador, western and south-central Bolivia, northwest Argentina, the southwesternmost tip of Colombia and a large portion of modern-day Chile, forming a state comparable to the historical empires of Eurasia. Its official language was Quechua. The Inca Empire was unique in that it lacked many of the features associated with civilization in the Old World. The anthropologist Gordon McEwan wrote that the Incas were able to construct "one of the greatest imperial states in human history" without the use of the wheel, draft animals, knowledge of iron or steel, or even a system of writing. Notable features of the Inca Empire included its monumental architecture, especially stonework, extensive road network (Qhapaq Ñan) reaching all corners of the empire, finely-woven textiles, use of knotted strings (quipu or khipu) for record keeping and communication, agricultural innovations and production in a difficult environment, and the organization and management fostered or imposed on its people and their labor. The Inca Empire functioned largely without money and without markets. Instead, exchange of goods and services was based on reciprocity between individuals and among individuals, groups, and Inca rulers. "Taxes" consisted of a labour obligation of a person to the Empire. The Inca rulers (who theoretically owned all the means of production) reciprocated by granting access to land and goods and providing food and drink in celebratory feasts for their subjects. Many local forms of worship persisted in the empire, most of them concerning local sacred huacas or wak'a, but the Inca leadership encouraged the sun worship of Inti – their sun god – and imposed its sovereignty above other religious groups, such as that of Pachamama. The Incas considered their king, the Sapa Inca, to be the "son of the Sun". The Inca economy has been the subject of scholarly debate. Darrell E. La Lone, in his work The Inca as a Nonmarket Economy, noted that scholars have previously described it as "feudal, slave, [or] socialist", as well as "a system based on reciprocity and redistribution; a system with markets and commerce; or an Asiatic mode of production."

Ancient Egypt was a cradle of civilization concentrated along the lower reaches of the Nile River in the eastern corner of North Africa. It emerged from prehistoric Egypt around 3150 BC (according to conventional Egyptian chronology), when Upper and Lower Egypt were united by Menes, who is believed by the majority of Egyptologists to have been the same person as Narmer. The history of ancient Egypt unfolded as a series of stable kingdoms interspersed by the "Intermediate Periods" of relative instability. These stable kingdoms existed in one of three periods: the Old Kingdom of the Early Bronze Age; the Middle Kingdom of the Middle Bronze Age; or the New Kingdom of the Late Bronze Age. The pinnacle of ancient Egyptian power was achieved during the New Kingdom, which extended its rule to much of Nubia and a considerable portion of the Levant. After this period, Egypt entered an era of slow decline. Over the course of its history, it was invaded or conquered by a number of foreign civilizations, including the Hyksos, the Kushites, the Assyrians, the Persians, and the Greeks and then the Romans. The end of ancient Egypt is variously defined as occurring with the end of the Late Period during the Wars of Alexander the Great in 332 BC or with the end of the Greek-ruled Ptolemaic Kingdom during the Roman conquest of Egypt in 30 BC. In AD 642, the Arab conquest of Egypt brought an end to the region's millennium-long Greco-Roman period. The success of ancient Egyptian civilization came partly from its ability to adapt to the Nile's conditions for agriculture. The predictable flooding of the Nile and controlled irrigation of its fertile valley produced surplus crops which supported a more dense population and thereby substantial social and cultural development. With resources to spare, the administration sponsored the mineral exploitation of the valley and its surrounding desert regions, the early development of an independent writing system, the organization of collective construction and agricultural projects, trade with other civilizations, and a military to assert Egyptian dominance throughout the Near East. Motivating and organizing these activities was a bureaucracy of elite scribes, religious leaders, and administrators under the control of the reigning pharaoh, who ensured the cooperation and unity of the Egyptian people in the context of an elaborate system of religious beliefs. Among the many achievements of ancient Egypt are: the quarrying, surveying, and construction techniques that supported the building of monumental pyramids, temples, and obelisks; a system of mathematics; a practical and effective system of medicine; irrigation systems and agricultural production techniques; the first known planked boats; Egyptian faience and glass technology; new forms of literature; and the earliest known peace treaty, which was ratified with the Anatolia-based Hittite Empire. Its art and architecture were widely copied and its antiquities were carried off to be studied, admired, or coveted in the far corners of the world. Likewise, its monumental ruins inspired the imaginations of travelers and writers for millennia. A newfound European and Egyptian respect for antiquities and excavations that began in earnest in the early modern period has led to much scientific investigation of ancient Egypt and its society, as well as a greater appreciation of its cultural legacy.

Ancient Greece (Ancient Greek: Ἑλλάς, romanized: Hellás) was a northeastern Mediterranean civilization, existing from the Greek Dark Ages of the 12th–9th centuries BC to the end of classical antiquity (c. 600 AD), comprising a loose collection of culturally and linguistically related city-states and communities. Prior to the Roman period, most of these regions were officially unified only once under the Kingdom of Macedon from 338 to 323 BC. In Western history, the era of classical antiquity was immediately followed by the Early Middle Ages and the Byzantine period. Three centuries after the decline of Mycenaean Greece during the Bronze Age collapse, Greek urban poleis began to form in the 8th century BC, ushering in the Archaic period and the colonization of the Mediterranean Basin. This was followed by the age of Classical Greece, from the Greco-Persian Wars to the death of Alexander the Great in 323 BC, and which included the Golden Age of Athens and the Peloponnesian War between Athens and Sparta. The unification of Greece by Macedon under Philip II and subsequent conquest of the Achaemenid Empire by Alexander the Great spread Hellenistic civilization across the Middle East. The Hellenistic period is considered to have ended in 30 BC, when the last Hellenistic kingdom, Ptolemaic Egypt, was annexed by the Roman Republic. Classical Greek culture, especially philosophy, had a powerful influence on ancient Rome, which carried a version of it throughout the Mediterranean and much of Europe. For this reason, Classical Greece is generally considered the cradle of Western civilization, the seminal culture from which the modern West derives many of its founding archetypes and ideas in politics, philosophy, science, and art.

The Mongol Empire was the largest contiguous empire in history. Originating in present-day Mongolia in East Asia, the medieval empire at its height stretched from the Sea of Japan to Eastern Europe, extending northward into Siberia and east and southward into the Indian subcontinent, mounting invasions of Southeast Asia, and conquering the Iranian Plateau; and reaching westward as far as the Levant and the Carpathian Mountains. The empire emerged from the unification of several nomadic tribes in the Mongol heartland under the leadership of Temüjin, known by the title of Genghis Khan (c. 1162–1227), whom a council proclaimed as the ruler of all Mongols in 1206. The empire grew rapidly under his rule and that of his descendants, who sent out invading armies in every direction. The vast transcontinental empire connected the East with the West, and the Pacific to the Mediterranean, in an enforced Pax Mongolica, allowing the exchange of trade, technologies, commodities, and ideologies across Eurasia. The empire began to split due to wars over succession, as the grandchildren of Genghis Khan disputed whether the royal line should follow from his son and initial heir Ögedei, or from one of his other sons, such as Tolui, Chagatai, or Jochi. The Toluids prevailed after a bloody purge of the Ögedeid and Chagatayid factions, but disputes continued among the descendants of Tolui. The conflict over whether the empire would adopt a sedentary, cosmopolitan lifestyle, or continue its nomadic, steppe-based way of life was a major factor in the breakup. After Möngke Khan died in 1259, rival kurultai councils simultaneously elected different successors, the brothers Ariq Böke and Kublai Khan, who fought each other in the Toluid Civil War (1260–1264) and dealt with challenges from the descendants of other sons of Genghis. Kublai successfully took power, but war ensued as he sought unsuccessfully to regain control of the Chagatayid and Ögedeid families. By Kublai's death in 1294, the empire had fractured into four separate khanates or empires, each pursuing its own objectives: the Golden Horde khanate in the northwest, the Chagatai Khanate in Central Asia, the Ilkhanate in Iran, and the Yuan dynasty in China, based in modern-day Beijing. In 1304, during the reign of Temür, the three western khanates accepted the suzerainty of the Yuan dynasty. The Ilkhanate was the first to fall, which disintegrated between 1335 and 1353. Next, the Yuan dynasty lost control of the Tibetan Plateau and China proper in 1354 and 1368, respectively, and collapsed after its capital Dadu was taken over by Ming forces. The Genghisid rulers of the Yuan then retreated north and continued to rule the Mongolian Plateau. The regime is thereafter known as the Northern Yuan dynasty, surviving as a rump state until the conquest by the Qing dynasty in the 1630s. The Golden Horde had broken into competing khanates by the end of the 15th century, and its rule on Eastern Europe is traditionally considered to have ended in 1480 with the Great Stand on the Ugra River by the Grand Duchy of Moscow, while the Chagatai Khanate lasted until 1687, or, in the Yarkent Khanate's case, until 1705.

The Industrial Revolution, sometimes called the First Industrial Revolution in contrast to the subsequent Second Industrial Revolution, was a transitional period of the global economy toward more widespread, efficient and stable manufacturing processes, succeeding the Second Agricultural Revolution. Beginning in Great Britain around 1760, the Industrial Revolution had spread to continental Europe and the United States by about 1840. Economic historians agree that the onset of the Industrial Revolution is the most important event in human history, comparable only to the adoption of agriculture with respect to material advancement. This transition included going from hand production methods to machines; new chemical manufacturing and iron production processes; the increasing use of water power and steam power; the development of machine tools; and rise of the mechanised factory system. Output greatly increased, and the result was an unprecedented rise in population and population growth. Many technological and architectural innovations were British. By the mid-18th century, Britain was the leading commercial nation, with GDP per capita considerably over the world average. The textile industry was the first to use modern production methods, and textiles became the dominant industry in terms of employment, value of output, and capital invested. The precise start and end of the Industrial Revolution is debated among historians, as is the pace of economic and social changes. Rapid adoption of mechanized textiles spinning occurred in Britain in the 1780s, and high rates of growth in steam power and iron production occurred after 1800. Mechanised textile production spread from Britain to continental Europe and the US in the early 19th century. A recession occurred from the late 1830s when the adoption of the Industrial Revolution's early innovations, such as mechanised spinning and weaving, slowed as markets matured despite increased adoption of locomotives, steamships, and hot blast iron smelting. New technologies such as the electrical telegraph, widely introduced in the 1840s in the UK and US, were not sufficient to drive high rates of growth. Rapid growth reoccurred after 1870, springing from new innovations in the Second Industrial Revolution. These included steel-making processes, mass production, assembly lines, electrical grid systems, large-scale manufacture of machine tools, and use of advanced machinery in steam-powered factories. The Industrial Revolution influenced almost every aspect of life. In particular, average income and population began to exhibit unprecedented sustained growth, with the standard of living improving for most in the Western world, though others have said it did not begin to improve meaningfully until the 20th century. Prior to the Industrial Revolution, most manufacturing occurred in China and India; after the Industrial Revolution, most manufacturing took place in North America and Western Europe.

The Renaissance (UK: rin-AY-sənss, US: REN-ə-sahnss) was a European period of history and cultural movement taking place at the end of the Late Middle Ages and the beginning of the early modern era. It is variously defined as lasting from the 14th century to the 17th or, more narrowly, as covering only the 15th and 16th centuries. It was characterized by the European rediscovery and revival of the literary, philosophical, and artistic achievements of classical antiquity. Associated with great change in art, architecture, politics, literature, exploration and technology, the Renaissance was first centered in the Republic of Florence, then spread to the rest of Italy and later throughout Europe. The Renaissance's intellectual basis was founded in its version of humanism, derived from the concept of Roman humanitas and the rediscovery of classical Greek philosophy, such as that of Protagoras, who said that "man is the measure of all things". Although the invention of metal movable type sped the dissemination of ideas from the later 15th century, the changes of the Renaissance were not uniform across Europe: the first traces appear in Italy as early as the late 13th century, in particular with the writings of Dante and the paintings of Giotto. As a cultural movement, the Renaissance encompassed innovative flowering of literary Latin and an explosion of vernacular literatures, beginning with the 14th-century resurgence of learning based on classical sources, which contemporaries credited to Petrarch; the development of linear perspective and other techniques of rendering a more natural reality in painting; and gradual but widespread educational reform. It saw myriad artistic developments and contributions from such polymaths as Leonardo da Vinci and Michelangelo, who inspired the term "Renaissance man". In politics, the Renaissance contributed to the development of the customs and conventions of diplomacy. The period also saw revolutions in printing and navigation, as well as the introduction of modern banking and the field of accounting.

The Hundred Years' War (French: Guerre de Cent Ans; 1337–1453 [116 years]) was a conflict between the kingdoms of England and France and a civil war in France during the late Middle Ages. It emerged from feudal disputes over the Duchy of Aquitaine and was triggered by a claim to the French throne made by Edward III of England. The war grew into a broader military, economic, and political struggle involving factions from across Western Europe, fuelled by emerging nationalism on both sides. The periodisation of the war typically charts it as taking place over 116 years. However, it was an intermittent conflict which was frequently interrupted by external factors, such as the Black Death, and several years of truces. The Hundred Years' War was a significant conflict in the Middle Ages. During the war, five generations of kings from two rival dynasties fought for the throne of France, then the wealthiest and most populous kingdom in Western Europe. The war had a lasting effect on European history: both sides produced innovations in military technology and tactics, including professional standing armies and artillery, that permanently changed European warfare. Chivalry reached its height during the conflict and subsequently declined. Stronger national identities took root in both kingdoms, which became more centralized and gradually emerged as global powers. The term "Hundred Years' War" was adopted by later historians and included dynastically related conflicts in Spain and Belgium, resulting in the longest military conflict in European history. The war is commonly divided into three phases, separated by truces: the Edwardian War (1337–1360), the Caroline War (1369–1389), and the Lancastrian War (1415–1453). Each side drew many allies into the conflict, with English forces initially prevailing; however, the French forces under the House of Valois ultimately retained control over the Kingdom of France. The French and English monarchies thereafter remained separate, despite the monarchs of England and Great Britain styling themselves as sovereigns of France until 1802.

The Thirty Years' War, fought primarily in Central Europe between 1618 and 1648, was one of the most destructive conflicts in European history. An estimated 4.5 to 8 million soldiers and civilians died from the effects of battle, famine, or disease, with parts of Germany reporting population declines of over 50%. Related conflicts include the Eighty Years' War, the War of the Mantuan Succession, the Franco-Spanish War, the Torstenson War, the Dutch–Portuguese War, and the Portuguese Restoration War. Its causes derived from religious conflict within the Holy Roman Empire, sparked by the 16th-century Reformation. The 1555 Peace of Augsburg attempted to resolve this by dividing the Empire into Catholic and Lutheran states, but was later destabilised by the expansion of Protestantism beyond these boundaries. Combined with disagreements over the limits of imperial authority, religion was thus an important factor in starting the war. However, its scope and extent were largely the consequence of external drivers such as the French–Habsburg rivalry and the Dutch Revolt. The war began in 1618, when the Catholic Emperor Ferdinand II was replaced as king of Bohemia by the Protestant Frederick V of the Palatinate. Although Frederick was swiftly deposed, his participation meant fighting expanded into the Palatinate. Their strategic importance drew in the Dutch Republic and Spain, then engaged in the Eighty Years' War, while the acquisition of imperial territories gave rulers like Christian IV of Denmark and Gustavus Adolphus of Sweden ongoing motives to intervene. These factors, together with Protestant fears that their religion was threatened, transformed an internal dynastic dispute into a wider conflict. The period from 1618 to 1635 was primarily a civil war within the Holy Roman Empire, which largely ended with the Peace of Prague. However, France's entry into the war in alliance with Sweden turned the empire into one theatre of a wider struggle with their Habsburg rivals, Emperor Ferdinand III and Spain. Fighting ended with the 1648 Peace of Westphalia, whose terms included greater autonomy for states like Bavaria and Saxony, Swedish territorial gains in northern Germany, as well as acceptance of Dutch independence by Spain. The conflict shifted the balance of power in favour of France and its subsequent expansion under Louis XIV.

The French Revolution was a period of political and societal change in France that began with the Estates General of 1789 and ended with the Coup of 18 Brumaire on 9 November 1799. Many of the revolution's ideas are considered fundamental principles of liberal democracy, and its values remain central to modern French political discourse. It was caused by a combination of social, political, and economic factors which the existing regime proved unable to manage. Financial crisis and widespread social distress led to the convocation of the Estates General in May 1789, its first meeting since 1614. The representatives of the Third Estate broke away and re-constituted themselves as a National Assembly in June. The Storming of the Bastille in Paris on 14 July led to a series of radical measures by the Assembly, including the abolition of feudalism, state control over the Catholic Church in France, and issuing the Declaration of the Rights of Man and of the Citizen. The next three years were dominated by a struggle for political control. King Louis XVI's attempted flight to Varennes in June 1791 further discredited the monarchy, and military defeats after the outbreak of the French Revolutionary Wars in April 1792 led to the insurrection of 10 August 1792. As a result, the monarchy was replaced by the French First Republic in September, followed by the execution of Louis XVI himself in January 1793. After another revolt in June 1793, the constitution was suspended, and political power passed from the National Convention to the Committee of Public Safety, dominated by radical Jacobins led by Maximilien Robespierre. About 16,000 people were sentenced by the Revolutionary Tribunal and executed in the Reign of Terror, which ended in July 1794 with the Thermidorian Reaction. Weakened by external threats and internal opposition, the Committee of Public Safety was replaced in November 1795 by the Directory. Its instability ended in 1799 with the coup of 18 Brumaire and the establishment of the Consulate, with Napoleon Bonaparte as First Consul.

Magna Carta (Medieval Latin for "Great Charter"), sometimes spelled Magna Charta, is a royal charter of rights sealed by King John of England at Runnymede, near Windsor, on 15 June 1215. First drafted by the Archbishop of Canterbury, Cardinal Stephen Langton, to make peace between the unpopular king and a group of rebel barons who demanded that the King confirm the Charter of Liberties, it promised the protection of church rights, protection for the barons from illegal imprisonment, access to swift and impartial justice, and limitations on feudal payments to the Crown, to be implemented through a council of 25 barons. Neither side stood by their commitments, and the charter was annulled by Pope Innocent III, leading to the First Barons' War. After John's death, the regency government of his young son, Henry III, reissued the document in 1216, stripped of some of its more radical content, in an unsuccessful bid to build political support for their cause. At the end of the war in 1217, it formed part of the peace treaty agreed at Lambeth, where the document acquired the name "Magna Carta", to distinguish it from the smaller Charter of the Forest, which was issued at the same time. Short of funds, Henry reissued the charter again in 1225 in exchange for a grant of new taxes. His son, Edward I, repeated the exercise in 1297, this time confirming it as part of England's statute law. However, Magna Carta was not unique; other legal documents of its time, both in England and beyond, made broadly similar statements of rights and limitations on the powers of the Crown. The charter became part of English political life and was typically renewed by each monarch in turn. As time went by and the fledgling Parliament of England passed new laws, it lost some of its practical significance. At the end of the 16th century, there was an upsurge of interest in Magna Carta. Lawyers and historians at the time believed that there was an ancient English constitution, going back to the days of the Anglo-Saxons, that protected individual English freedoms. They argued that the Norman invasion of 1066 had overthrown these rights and that Magna Carta had been a popular attempt to restore them, making the charter an essential foundation for the contemporary powers of Parliament and legal principles such as habeas corpus. Although this historical account was badly flawed, jurists such as Sir Edward Coke invoked Magna Carta extensively in the early 17th century, arguing against the divine right of kings. Both James I and his son Charles I attempted to suppress the discussion of Magna Carta. The political myth of Magna Carta as a medieval revival of ancient personal liberties persisted after the Glorious Revolution of 1688 until well into the 19th century. It influenced the early American colonists in the Thirteen Colonies and the formation of the United States Constitution, which became the supreme law of the land in the new republic of the United States. Research by Victorian historians showed that the original 1215 charter had concerned the medieval relationship between the monarch and the barons, and not ordinary subjects. The majority of historians now see the interpretation of the charter as a unique and early charter of universal legal rights as a myth that was created centuries later. Despite the changes in views of historians, the charter has remained a powerful, iconic document, even after almost all of its content was repealed from the statute books in the 19th and 20th centuries. Magna Carta still forms an important symbol of liberty today, often cited by politicians and campaigners, and is held in great respect by the British and American legal communities, Lord Denning describing it in 1956 as "the greatest constitutional document of all times—the foundation of the freedom of the individual against the arbitrary authority of the despot". In the 21st century, four exemplifications of the original 1215 charter remain in existence, two at the British Library, one at Lincoln Castle and one at Salisbury Cathedral. These are recognised by UNESCO on its Memory of the World international register. There are also a handful of the subsequent charters in public and private ownership, including copies of the 1297 charter in both the United States and Australia. The 800th anniversary of Magna Carta in 2015 included extensive celebrations and discussions, and the four original 1215 charters were displayed together at the British Library. None of the original 1215 Magna Carta is currently in force since it has been repealed; however, three clauses of the original charter are enshrined in the 1297 reissued Magna Carta and do still remain in force in England and Wales.

The Viking Age (about 800–1050 CE) was the period during the Middle Ages when Norsemen known as Vikings undertook large-scale raiding, colonising, conquest, and trading throughout Europe and reached North America. The Viking Age applies not only to their homeland of Scandinavia but also to any place significantly settled by Scandinavians during the period. Although few of the Scandinavians of the Viking Age were Vikings in the sense of being engaged in piracy, they are often referred to as Vikings as well as Norsemen. Voyaging by sea from their homelands in Denmark, Norway, and Sweden, the Norse people settled in the British Isles, Ireland, the Faroe Islands, Iceland, Greenland, Normandy, and the Baltic coast and along the Dnieper and Volga trade routes in eastern Europe, where they were also known as Varangians. They also briefly settled in Newfoundland, becoming the first Europeans to reach North America. The Norse-Gaels, Normans, Rus' people, Faroese, and Icelanders emerged from these Norse colonies. The Vikings founded several kingdoms and earldoms in Europe: the Kingdom of the Isles (Suðreyjar), Orkney (Norðreyjar), York (Jórvík) and the Danelaw (Danalǫg), Dublin (Dyflin), Normandy, and Kievan Rus' (Garðaríki). The Norse homelands were also unified into larger kingdoms during the Viking Age, and the short-lived North Sea Empire included large swathes of Scandinavia and Britain. In 1021, the Vikings achieved the feat of reaching North America—the date of which was not determined until a millennium later. Several factors drove this expansion. The Vikings were drawn by the growth of wealthy towns and monasteries overseas and weak kingdoms. They may also have been pushed to leave their homeland by overpopulation, lack of good farmland, and political strife arising from the unification of Norway. The aggressive expansion of the Carolingian Empire and forced conversion of the neighbouring Saxons to Christianity may also have been a factor. Sailing innovations had allowed the Vikings to sail farther and longer to begin with. Information about the Viking Age is drawn largely from primary sources written by those the Vikings encountered, as well as archaeology, supplemented with secondary sources such as the Icelandic Sagas.

Penicillins (P, PCN or PEN) are a group of β-lactam antibiotics originally obtained from Penicillium moulds, principally P. chrysogenum and P. rubens. Most penicillins in clinical use are synthesised by P. chrysogenum using deep tank fermentation and then purified. A number of natural penicillins have been discovered, but only two purified compounds are in clinical use: penicillin G (intramuscular or intravenous use) and penicillin V (given by mouth). Penicillins were among the first medications to be effective against many bacterial infections caused by staphylococci and streptococci. They are still widely used today for various bacterial infections, though many types of bacteria have developed resistance following extensive use. In the United States, 10% of the population claims penicillin allergies, but because the frequency of positive skin test results decreases by 10% with each year of avoidance, 90% of these patients can eventually tolerate penicillin. Additionally, those with penicillin allergies can usually tolerate cephalosporins (another group of β-lactam) because the immunoglobulin E (IgE) cross-reactivity is only 3%. Penicillin was discovered in 1928 by the Scottish physician Alexander Fleming as a crude extract of P. rubens. Fleming's student Cecil George Paine was the first to successfully use penicillin to treat eye infection (neonatal conjunctivitis) in 1930. The purified compound (penicillin F) was isolated in 1940 by a research team led by Howard Florey and Ernst Boris Chain at the University of Oxford. Fleming first used the purified penicillin to treat streptococcal meningitis in 1942. The 1945 Nobel Prize in Physiology or Medicine was shared by Chain, Fleming and Florey. Several semisynthetic penicillins are effective against a broader spectrum of bacteria: these include the antistaphylococcal penicillins, aminopenicillins, and antipseudomonal penicillins.

Alzheimer's disease (AD) is a neurodegenerative disease and is the most common form of dementia, accounting for around 60–70% of cases. The most common early symptom is difficulty in remembering recent events. As the disease advances, symptoms can include problems with language, disorientation (including easily getting lost), mood swings, loss of motivation, self-neglect, and behavioral issues. As a person's condition declines, they often withdraw from family and society. Gradually, bodily functions are lost, ultimately leading to death. Although the speed of progression can vary, the average life expectancy following diagnosis is three to twelve years. The causes of Alzheimer's disease remain poorly understood. There are many environmental and genetic risk factors associated with its development. The strongest genetic risk factor is from an allele of apolipoprotein E, a protein involved in the metabolism of fats in mammals. Other risk factors include a history of head injury, clinical depression, and high blood pressure. The progression of the disease is largely characterised by the accumulation of malformed protein deposits in the cerebral cortex, called amyloid plaques and neurofibrillary tangles. These misfolded protein aggregates interfere with normal cell function, and over time lead to irreversible degeneration of neurons and loss of synaptic connections in the brain. A probable diagnosis is based on the history of the illness and cognitive testing, with medical imaging and blood tests to rule out other possible causes. Initial symptoms are often mistaken for normal brain aging. Examination of brain tissue at postmortem used to be the only way to definitively diagnose Alzheimer's but neuroimaging techniques and fluid biomarkers have made diagnosis in vivo possible. No treatments can stop or reverse its progression, though some may temporarily improve symptoms. A healthy diet, physical activity, and social engagement are generally beneficial in aging, and may help in reducing the risk of cognitive decline and Alzheimer's. Affected people become increasingly reliant on others for assistance, often placing a burden on caregivers. The pressures can include social, psychological, physical, and economic elements. Exercise programs may be beneficial with respect to activities of daily living and can potentially improve outcomes. Behavioral problems or psychosis due to dementia are sometimes treated with antipsychotics, but this has an increased risk of early death. As of 2020, there were approximately 50 million people worldwide with Alzheimer's disease. It most often begins in people over 65 years of age, although up to 10% of cases are early-onset impacting those in their 30s to mid-60s. It affects about 6% of people 65 years and older, and women more often than men. The disease is named after German psychiatrist and pathologist Alois Alzheimer, who first described it in 1906. Alzheimer's financial burden on society is large, with an estimated global annual cost of US$1 trillion. Alzheimer's and related dementias are ranked as the seventh leading cause of death worldwide. Given the widespread impacts of Alzheimer's disease, both basic-science and health funders in many countries support Alzheimer's research at large scales. For example, the US National Institutes of Health program for Alzheimer's research, the National Plan to Address Alzheimer's Disease, has a budget of US$3.98 billion for fiscal year 2026. In the European Union, the 2020 Horizon Europe research programme awarded over €570 million for dementia-related projects.

The human immunodeficiency virus (HIV) is a retrovirus that attacks the immune system. Without treatment, it can lead to a spectrum of conditions including acquired immunodeficiency syndrome (AIDS). It is a preventable disease. It can be managed with treatment and become a manageable chronic health condition. While there is no cure or vaccine for HIV, antiretroviral treatment can slow the course of the disease, and, if used before significant disease progression, can extend the life expectancy of someone living with HIV to a nearly standard level. An HIV-positive person on treatment can expect to live a normal life, and die with the virus, not of it. Effective treatment for HIV-positive people (people living with HIV) involves a life-long regimen of medicine to suppress the virus, making the viral load undetectable. Early testing can show if treatment is needed to stop progression and to prevent infecting others. Treatment is recommended as soon as the diagnosis is made. An HIV-positive person who has an undetectable viral load as a result of long-term treatment has effectively no risk of transmitting HIV sexually. Campaigns by UNAIDS and organizations around the world have communicated this as Undetectable = Untransmittable. Without treatment the infection can interfere with the immune system, and eventually progress to AIDS, sometimes taking many years. Following initial infection an individual may not notice any symptoms, or may experience a brief period of influenza-like illness. During this period the person may not know that they are HIV-positive, yet they will be able to pass on the virus. Typically, this period is followed by a prolonged incubation period with no symptoms. Eventually the HIV infection increases the risk of developing other infections such as tuberculosis, as well as other opportunistic infections, and tumors which are rare in people who have normal immune function. The late stage is often also associated with unintended weight loss. Without treatment, the life expectancy of an individual with HIV is around 11 years. HIV is spread primarily by unprotected sex (including anal, oral and vaginal sex), contaminated hypodermic needles or blood transfusions, and from mother to child during pregnancy, delivery, or breastfeeding. Some bodily fluids, such as saliva, sweat, and tears, do not transmit the virus. Oral sex has little risk of transmitting the virus. Ways to avoid catching HIV and preventing the spread include safe sex, treatment to prevent infection ("PrEP"), treatment to stop infection in someone who has been recently exposed ("PEP"), treating those who are infected, and needle exchange programs. Disease in a baby can often be prevented by giving both the mother and child antiretroviral medication. Recognized worldwide in the early 1980s, HIV/AIDS has had a large impact on society, both as an illness and as a source of discrimination. The disease also has large economic impacts. There are many misconceptions about HIV/AIDS, such as the belief that it can be transmitted by casual non-sexual contact. The disease has become subject to many controversies involving religion, including the Catholic Church's position not to support condom use as prevention. It has attracted international medical and political attention as well as large-scale funding since it was identified in the 1980s. HIV made the jump from other primates to humans in west-central Africa in the early-to-mid-20th century. AIDS was first recognized by the U.S. Centers for Disease Control and Prevention (CDC) in 1981 and its cause—HIV infection—was identified in the early part of the decade. Between the first time AIDS was readily identified through 2024, the disease is estimated to have caused at least 42.3 million deaths worldwide. In 2023, 630,000 people died from HIV-related causes, an estimated 1.3 million people acquired HIV and about 39.9 million people worldwide living with HIV, 65% of whom are in the World Health Organization (WHO) African Region. HIV/AIDS is considered a pandemic—a disease outbreak which is present over a large area and is actively spreading. The United States' National Institutes of Health (NIH) and the Gates Foundation have pledged $200 million focused on developing a global cure for AIDS.

Cancer is a group of diseases involving uncontrolled cell growth typically resulting in tumors with the potential to invade or spread to other parts of the body. These malignant tumors contrast with benign tumors, which do not spread. Over 100 types of cancers affect humans. About 33% of deaths from cancer are caused by tobacco and alcohol, obesity, lack of fruit and vegetables in the diet and lack of exercise. Other factors include certain infections, exposure to ionizing radiation, and environmental pollutants. Infection with specific viruses, bacteria, and parasites causes approximately 16–18% of cancers worldwide. These infectious agents include Helicobacter pylori, hepatitis B, hepatitis C, and HPV. Human immunodeficiency virus (HIV) does not directly cause cancer, but it causes immune deficiency that can increase the risk of cancer from other infections, sometimes up to several thousandfold. Vaccination against the hepatitis B virus and the human papillomavirus have been shown to nearly eliminate the risk of cancers caused by these viruses. These environmental factors act, at least partly, by changing the genes of a cell. Typically, many genetic changes are required before cancer develops. Approximately 5–10% of cancers are due to inherited genetic defects. Cancer can be detected by certain signs and symptoms or screening tests. It is then typically further investigated by medical imaging and confirmed by biopsy. The risk of developing cancer can be reduced by not smoking, maintaining a healthy weight, limiting alcohol intake, eating plenty of vegetables, fruits, and whole grains, vaccination against certain infectious diseases, limiting consumption of processed meat and red meat, and limiting exposure to direct sunlight. Early detection through screening is useful for cervical and colorectal cancer. The benefits of screening for breast cancer are controversial. Cancer is often treated with some combination of radiation therapy, surgery, chemotherapy and targeted therapy. More personalized therapies that harness a patient's immune system are emerging in the field of cancer immunotherapy. Palliative care is a medical specialty that delivers advanced pain and symptom management, which may be particularly important in those with advanced disease. The chance of survival depends on the type of cancer and extent of disease at the start of treatment. In children under 15 at diagnosis, the five-year survival rate in the developed world is on average 80%. For cancer in the United States, the average five-year survival rate is 66% for all ages. In 2015, about 90.5 million people worldwide had cancer. In 2019, annual cancer cases grew by 23.6 million people, and there were 10 million deaths worldwide, representing over the previous decade increases of 26% and 21%, respectively. The most common types of cancer in males are lung cancer, prostate cancer, colorectal cancer, and stomach cancer. In females, the most common types are breast cancer, colorectal cancer, lung cancer, and cervical cancer. If skin cancer other than melanoma were included in total new cancer cases each year, it would account for around 40% of cases. In children, acute lymphoblastic leukemia and brain tumors are most common, except in Africa, where non-Hodgkin lymphoma occurs more often. In 2012, about 165,000 children under 15 years of age were diagnosed with cancer. The risk of cancer increases significantly with age, and many cancers occur more commonly in developed countries. Rates are increasing as more people live to an old age and as lifestyle changes occur in the developing world. The global total economic costs of cancer were estimated at US$1.16 trillion (equivalent to $1.71 trillion in 2025) per year as of 2010.

The human brain is the central organ of the nervous system, and with the spinal cord, comprises the central nervous system. It consists of the cerebrum, the brainstem and the cerebellum. The brain controls most of the activities of the body, processing, integrating, and coordinating the information it receives from the sensory nervous system. The brain integrates sensory information and coordinates instructions sent to the rest of the body. The cerebrum, the largest part of the human brain, consists of two cerebral hemispheres. Each hemisphere has an inner core composed of white matter, and an outer surface – the cerebral cortex – composed of grey matter. The cortex has an outer layer, the neocortex, and an inner allocortex. The neocortex is made up of six neuronal layers, while the allocortex has three or four. Each hemisphere is divided into four lobes – the frontal, parietal, temporal, and occipital lobes. The frontal lobe is associated with executive functions including self-control, planning, reasoning, and abstract thought, while the occipital lobe is dedicated to vision. Within each lobe, cortical areas are associated with specific functions, such as the sensory, motor, and association regions. Although the left and right hemispheres are broadly similar in shape and function, some functions are associated with one side, such as language in the left and visual-spatial ability in the right. The hemispheres are connected by commissural nerve tracts, the largest being the corpus callosum. The cerebrum is connected by the brainstem to the spinal cord. The brainstem consists of the midbrain, the pons, and the medulla oblongata. The cerebellum is connected to the brainstem by three pairs of nerve tracts called cerebellar peduncles. Within the cerebrum is the ventricular system, consisting of four interconnected ventricles in which cerebrospinal fluid is produced and circulated. Underneath the cerebral cortex are several structures, including the thalamus, the epithalamus, the pineal gland, the hypothalamus, the pituitary gland, and the subthalamus; the limbic structures, including the amygdalae and the hippocampi, the claustrum, the various nuclei of the basal ganglia, the basal forebrain structures, and three circumventricular organs. Brain structures that are not on the midplane exist in pairs; for example, there are two hippocampi and two amygdalae. The cells of the brain include neurons and supportive glial cells. There are more than 86 billion neurons in the brain, and a more or less equal number of other cells. Brain activity is made possible by the interconnections of neurons and their release of neurotransmitters in response to nerve impulses. Neurons connect to form neural pathways, neural circuits, and elaborate network systems. The whole circuitry is driven by the process of neurotransmission. The brain is protected by the skull, suspended in cerebrospinal fluid, and isolated from the bloodstream by the blood–brain barrier. However, the brain is still susceptible to damage, disease, and infection. Damage can be caused by trauma, or a loss of blood supply known as a stroke. The brain is susceptible to degenerative disorders, such as Parkinson's disease, dementias including Alzheimer's disease, and multiple sclerosis. Psychiatric conditions, including schizophrenia and clinical depression, are thought to be associated with brain dysfunctions. The brain can also be the site of tumours, both benign and malignant; these mostly originate from other sites in the body. The study of the anatomy of the brain is neuroanatomy, while the study of its function is neuroscience. Numerous techniques are used to study the brain. Specimens from other animals, which may be examined microscopically, have traditionally provided much information. Medical imaging technologies such as functional neuroimaging, and electroencephalography (EEG) recordings are important in studying the brain. The medical history of people with brain injury has provided insight into the function of each part of the brain. Neuroscience research has expanded considerably, and research is ongoing. In culture, the philosophy of mind has for centuries attempted to address the question of the nature of consciousness and the mind–body problem. The pseudoscience of phrenology attempted to localise personality attributes to regions of the cortex in the 19th century. In science fiction, brain transplants are imagined in tales such as the 1942 Donovan's Brain.

Insulin ( ; from Latin insula 'island') is a peptide hormone produced by beta cells of the pancreatic islets encoded in humans by the insulin (INS) gene. It is the main anabolic hormone of the body. It regulates the metabolism of carbohydrates, fats, and protein by promoting the absorption of glucose from the blood into cells of the liver, fat, and skeletal muscles. In these tissues the absorbed glucose is converted into either glycogen, via glycogenesis, or fats (triglycerides), via lipogenesis; in the liver, glucose is converted into both. Glucose production and secretion by the liver are strongly inhibited by high concentrations of insulin in the blood. Circulating insulin also affects the synthesis of proteins in a wide variety of tissues. It is thus an anabolic hormone, promoting the conversion of small molecules in the blood into large molecules in the cells. Low insulin in the blood has the opposite effect, promoting widespread catabolism, especially of reserve body fat. Beta cells are sensitive to blood sugar levels so that they secrete insulin into the blood in response to high level of glucose, and inhibit secretion of insulin when glucose levels are low. Insulin production is also regulated by glucose: high glucose promotes insulin production while low glucose levels lead to lower production. Insulin enhances glucose uptake and metabolism in the cells, thereby reducing blood sugar. Their neighboring alpha cells, by taking their cues from the beta cells, secrete glucagon into the blood in the opposite manner: increased secretion when blood glucose is low, and decreased secretion when glucose concentrations are high. Glucagon increases blood glucose by stimulating glycogenolysis and gluconeogenesis in the liver. The secretion of insulin and glucagon into the blood in response to the blood glucose concentration is the primary mechanism of glucose homeostasis. Decreased or absent insulin activity results in diabetes, a condition of high blood sugar level (hyperglycaemia). There are two types of the disease. In type 1 diabetes, the beta cells are destroyed by an autoimmune reaction so that insulin can no longer be synthesized or be secreted into the blood. In type 2 diabetes, the destruction of beta cells is less pronounced than in type 1, and is not due to an autoimmune process. Instead, there is an accumulation of amyloid in the pancreatic islets, which likely disrupts their anatomy and physiology. The pathogenesis of type 2 diabetes is not well understood but reduced population of islet beta-cells, reduced secretory function of islet beta-cells that survive, and peripheral tissue insulin resistance are known to be involved. Type 2 diabetes is characterized by increased glucagon secretion which is unaffected by, and unresponsive to the concentration of blood glucose. But insulin is still secreted into the blood in response to the blood glucose. As a result, glucose accumulates in the blood. The human insulin protein is composed of 51 amino acids, and has a molecular mass of 5808 Da. It is a heterodimer of an A-chain and a B-chain, which are linked together by disulfide bonds. Insulin's structure varies slightly between species of animals. Insulin from non-human animal sources differs somewhat in effectiveness (in carbohydrate metabolism effects) from human insulin because of these variations. Porcine insulin is especially close to the human version, and was widely used to treat type 1 diabetics before human insulin could be produced in large quantities by recombinant DNA technologies. Insulin was the first peptide hormone discovered. Frederick Banting and Charles Best, working in the laboratory of John Macleod at the University of Toronto, were the first to isolate insulin from dog pancreas in 1921. Frederick Sanger sequenced the amino acid structure in 1951, which made insulin the first protein to be fully sequenced. The crystal structure of insulin in the solid state was determined by Dorothy Hodgkin in 1969. Insulin is also the first protein to be chemically synthesised and produced by DNA recombinant technology. It is on the WHO Model List of Essential Medicines, the most important medications needed in a basic health system.

Coronavirus disease 2019 (COVID-19) is a contagious disease caused by the coronavirus SARS-CoV-2. Starting in January 2020, the disease spread worldwide, resulting in the COVID-19 pandemic. In March 2020, the World Health Organization declared COVID-19 a global health emergency; they declared the end of the emergency in May 2023. The symptoms of COVID‑19 can vary but often include fever, fatigue, cough, breathing difficulties, loss of smell, and loss of taste. Symptoms may begin one to fourteen days after exposure to the virus. At least a third of people who are infected do not develop noticeable symptoms. Of those who develop symptoms noticeable enough to be classified as patients, most (81%) develop mild to moderate symptoms (up to mild pneumonia), while 14% develop severe symptoms (dyspnea, hypoxia, or more than 50% lung involvement on imaging), and 5% develop critical symptoms (respiratory failure, shock, or multiorgan dysfunction). Older people have a higher risk of developing severe symptoms and dying. Some people experience persistent symptoms (long COVID), for months or years after infection, including fatigue, cognitive issues and shortness of breath. Damage to organs has been observed in a subset. COVID‑19 transmission occurs when infectious particles are breathed in or come into contact with the eyes, nose, or mouth. The risk is highest when people are in close proximity, but small airborne particles containing the virus can remain suspended in the air and travel over longer distances, particularly indoors. Transmission can also occur when people touch their eyes, nose, or mouth after touching surfaces or objects that have been contaminated by the virus. People remain contagious for up to 20 days and can spread the virus even if they do not develop symptoms. There are two common tests to detect a COVID infection. Antigen tests (also called rapid lateral flow tests) can be used at home. A positive test indicates an active infection. However, negative test results are not always accurate, especially when there are no symptoms. Health care providers can perform a more accurate PCR test, which is typically analysed in a laboratory. Several COVID-19 vaccines have been approved and distributed in various countries, many of which have initiated mass vaccination campaigns. Other preventive measures include physical or social distancing, quarantining, ventilation of indoor spaces, use of face masks or coverings in public, covering coughs and sneezes, hand washing, and keeping unwashed hands away from the face. Initial treatment consists of drugs that have been developed to inhibit the virus for those at high risk and symptomatic treament, managing the disease through supportive care. The first known case was identified in Wuhan, China, in December 2019. Most scientists believe that the SARS-CoV-2 virus entered into human populations through natural zoonosis, similar to the SARS-CoV-1 and MERS-CoV outbreaks, and consistent with other pandemics in human history. Social and environmental factors including climate change, natural ecosystem destruction and wildlife trade increased the likelihood of such zoonotic spillover.

A blood type (also known as a blood group) is a classification of blood based on the presence and absence of antibodies and inherited antigenic substances on the surface of red blood cells (RBCs). These antigens may be proteins, carbohydrates, glycoproteins, or glycolipids, depending on the blood group system. Some of these antigens are also present on the surface of other types of cells of various tissues. Several of these red blood cell surface antigens can stem from one allele (or an alternative version of a gene) and collectively form a blood group system. Blood types are inherited and represent contributions from both parents of an individual. As of June 2025, a total of 48 human blood group systems are recognized by the International Society of Blood Transfusion (ISBT). The two most important blood group systems are ABO and Rh; they determine someone's blood type (A, B, AB, and O, with + or − denoting RhD status) for suitability in blood transfusion.

Bacteria are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among the first life forms to appear on Earth, and are present in most of its habitats. Bacteria inhabit the air, soil, water, acidic hot springs, radioactive waste, and the deep biosphere of Earth's crust. Bacteria play a vital role in many stages of the nutrient cycle by recycling nutrients and the fixation of nitrogen from the atmosphere. The nutrient cycle includes the decomposition of dead bodies; bacteria are responsible for the putrefaction stage in this process. In the biological communities surrounding hydrothermal vents and cold seeps, extremophile bacteria provide the nutrients needed to sustain life by converting dissolved compounds, such as hydrogen sulphide and methane, to energy. Bacteria also live in mutualistic, commensal and parasitic relationships with plants and animals. Most bacteria have not been characterised and there are many species that cannot be grown in the laboratory. The study of bacteria is known as bacteriology, a branch of microbiology. Like all animals, humans carry vast numbers (approximately 1013 to 1014) of bacteria. Most are in the gut, though there are many on the skin. Most of the bacteria in and on the body are harmless or rendered so by the protective effects of the immune system, and many are beneficial, particularly the ones in the gut. However, several species of bacteria are pathogenic and cause infectious diseases, including cholera, syphilis, anthrax, leprosy, tuberculosis, tetanus and bubonic plague. The most common fatal bacterial diseases are respiratory infections. Antibiotics are used to treat bacterial infections and are also used in farming, making antibiotic resistance a growing problem. Bacteria are important in sewage treatment and the breakdown of oil spills, the production of cheese and yogurt through fermentation, the recovery of gold, palladium, copper and other metals in the mining sector (biomining, bioleaching), as well as in biotechnology, and the manufacture of antibiotics and other chemicals. Once regarded as plants constituting the class Schizomycetes ("fission fungi"), bacteria are now classified as prokaryotes. Unlike cells of animals and other eukaryotes, bacterial cells contain circular chromosomes, do not contain a nucleus and rarely harbour membrane-bound organelles. Although the term bacteria traditionally included all prokaryotes, the scientific classification changed after the discovery in the 1990s that prokaryotes consist of two very different groups of organisms that evolved from an ancient common ancestor. These evolutionary domains are called Bacteria and Archaea. Unlike Archaea, bacteria contain ester-linked lipids in the cell membrane, possess elongation factors that are resistant to ADP-ribosylation by diphtheria toxin, use formylmethionine in protein synthesis initiation, and have numerous genetic differences, including a different 16S rRNA.

Chemotherapy (often abbreviated chemo, sometimes CTX and CTx) is the type of cancer treatment that uses one or more anti-cancer drugs (chemotherapeutic agents or alkylating agents) in a standard regimen. Chemotherapy may be given with a curative intent (which almost always involves combinations of drugs), or it may aim only to prolong life or to reduce symptoms (palliative chemotherapy). Chemotherapy is one of the major categories of the medical discipline specifically devoted to pharmacotherapy for cancer, which is called medical oncology. The term chemotherapy now means the non-specific use of intracellular poisons to inhibit mitosis (cell division) or to induce DNA damage (so that DNA repair can augment chemotherapy). This meaning excludes the more-selective agents that block extracellular signals (signal transduction). Therapies with specific molecular or genetic targets, which inhibit growth-promoting signals from classic endocrine hormones (primarily estrogens for breast cancer and androgens for prostate cancer), are now called hormonal therapies. Other inhibitions of growth-signals, such as those associated with receptor tyrosine kinases, are targeted therapy. The use of drugs (whether chemotherapy, hormonal therapy, or targeted therapy) is systemic therapy for cancer: they are introduced into the blood stream (the system) and therefore can treat cancer anywhere in the body. Systemic therapy is often used with other, local therapy (treatments that work only where they are applied), such as radiation, surgery, and hyperthermia. Traditional chemotherapeutic agents are cytotoxic by means of interfering with cell division (mitosis) but cancer cells vary widely in their susceptibility to these agents. To a large extent, chemotherapy can be thought of as a way to damage or stress cells, which may then lead to cell death if apoptosis is initiated. Many of the side effects of chemotherapy can be traced to damage to normal cells that divide rapidly and are thus sensitive to anti-mitotic drugs: cells in the bone marrow, digestive tract and hair follicles. This results in the most common side-effects of chemotherapy: myelosuppression (decreased production of blood cells, hence that also immunosuppression), mucositis (inflammation of the lining of the digestive tract), and alopecia (hair loss). Because of the effect on immune cells (especially lymphocytes), chemotherapy drugs often find use in a host of diseases that result from harmful overactivity of the immune system against self (so-called autoimmunity). These include rheumatoid arthritis, systemic lupus erythematosus, multiple sclerosis, vasculitis, malignant hypertension and many others.

Pharmacology is the science of drugs and medications, including a substance's origin, composition and interaction with biological systems; specifically through pharmacokinetics, pharmacodynamics, therapeutic use, and toxicology. The discipline examines these interactions through pharmacokinetics (what the body does to the drug) and pharmacodynamics (what the drug does to the body), both of which determine how a substance alters normal or abnormal biochemical function. Substances with medicinal properties are classified as pharmaceuticals, while the term drug encompasses any chemical agent that alters biological processes. The field encompasses drug composition and properties, functions, sources, medicinal chemistry, drug design, molecular and cellular mechanisms, organ/systems mechanisms, signal transduction/cellular communication, molecular diagnostics, interactions, chemical biology, therapy, medical applications, toxicology, and antipathogenic capabilities. The two main areas of pharmacology are pharmacodynamics and pharmacokinetics. Pharmacodynamics studies the effects of a drug on biological systems, and pharmacokinetics studies the effects of biological systems on a drug. In broad terms, pharmacodynamics discusses the chemicals with biological receptors, and pharmacokinetics discusses the liberation, absorption, distribution, metabolism, and excretion (LADME) of chemicals from the biological systems. Pharmacology is not synonymous with pharmacy, though the two terms are frequently confused. Pharmacology is a branch of medical and biological sciences which encompasses the research, discovery, and characterization of chemicals exhibiting biological effects, alongside the elucidation of cellular and organismal function in relation to these chemicals. In contrast, pharmacy, a health services profession, is concerned with the application of the principles learned from pharmacology, pharmaceutics, medicinal chemistry, pharmacognosy, clinical pharmacy and others in its clinical settings; whether it be in a dispensing or clinical care role. In either field, the primary contrast between the two is their distinction between direct-patient care, pharmacy practice, and the science-oriented research field, driven by pharmacology.

The immune system is a network of biological systems that protects an organism from diseases. It detects and responds to a wide variety of pathogens, such as viruses, bacteria, and parasites, as well as cancer cells and foreign objects, such as wood splinters—distinguishing them from the organism's own healthy tissue. Many species have two major subsystems of the immune system. The innate immune system provides a preconfigured response to broad groups of situations and stimuli. The adaptive immune system provides a tailored response to each stimulus by learning to recognize molecules it has previously encountered. Both use molecules and cells to perform their functions. Nearly all organisms have some kind of immune system. Bacteria have a rudimentary immune system in the form of enzymes that protect against viral infections. Other basic immune mechanisms evolved in ancient plants and animals (eukaryotes) and remain in their modern descendants. These mechanisms include phagocytosis, antimicrobial peptides called defensins, RNA interference, and the complement system. Jawed vertebrates, including humans, have a more sophisticated defense mechanism. For example, adaptive immunity provides the ability to adapt to recognize pathogens more efficiently (somatic hypermutation). This process creates an immunological memory, resulting in an enhanced response against subsequent encounters with that same pathogen. This mechanism of adaptive immunity is the fundamental basis of vaccination. Dysfunction of the immune system can cause autoimmune diseases, inflammatory diseases and cancer. Immunodeficiency occurs when the immune system is less active than normal, resulting in recurring and life-threatening infections. In humans, immunodeficiency can be the result of a genetic disease such as severe combined immunodeficiency, acquired conditions such as HIV/AIDS, or the use of immunosuppressive medication. Autoimmunity results from a hyperactive immune system attacking normal tissues as if they were foreign organisms. Common autoimmune diseases include Hashimoto's thyroiditis, rheumatoid arthritis, diabetes mellitus type 1, and systemic lupus erythematosus. Immunology covers the study of all aspects of the immune system.

Diabetes mellitus, commonly known as diabetes, is a group of common endocrine diseases characterized by sustained high blood sugar levels. Diabetes tends to progress in severity, and is due to either a reduced production of the hormone insulin by the pancreas or unresponsiveness of bodily cells to insulin's effects. Classic symptoms include the three Ps: polydipsia (excessive thirst), polyuria (excessive urination), and polyphagia (excessive hunger), together with weight loss and blurred vision. If left untreated, the disease can lead to many health complications, including disorders of the cardiovascular system, eye, kidney, and nerves. The major types of diabetes are type 1 and type 2. The most common type is type 2. The most common treatment for type 1 is insulin replacement therapy (insulin injections), while anti-diabetic medications (such as metformin and semaglutide or tirzepatide) and lifestyle modifications can be used to manage type 2. Gestational diabetes, a form that sometimes arises during pregnancy, normally resolves shortly after delivery. Type 1 diabetes is an autoimmune condition in which the body's immune system attacks the beta cells (β-cell) in the pancreas, preventing the production of insulin. This condition typically is present from birth or develops early in life. Type 2 diabetes occurs when the body becomes resistant to insulin and thus, glucose remains in the bloodstream instead of being absorbed by the cells. Additionally, diabetes can result from other specific causes, such as genetic conditions (monogenic diabetes syndromes like neonatal diabetes and maturity-onset diabetes of the young), diseases affecting the pancreas (such as pancreatitis), or the use of certain medications and chemicals (such as glucocorticoids or other specific drugs, including those used after organ transplantation). The number of people worldwide diagnosed as living with diabetes has increased sharply in recent decades, from 200 million in 1990 to 828 million in 2024. Diabetes accounts for some two million deaths each year, and more than half of those affected are unaware of their condition. It affects one in seven of the adult human population, with type 2 diabetes accounting for more than 95% of cases. These numbers have already risen beyond earlier projections of 783 million adults by 2045. The prevalence of the disease continues to increase, most dramatically in low-middle income nations, where it is now the seventh leading cause of death. Rates are similar in women and men. The global expenditure on diabetes-related healthcare is an estimated US$760 billion a year.

Aspirin () is the genericized trademark for acetylsalicylic acid (ASA), a nonsteroidal anti-inflammatory drug (NSAID) used to reduce pain, fever, and inflammation, and as an antithrombotic. Aspirin is used to treat inflammatory conditions including Kawasaki disease, pericarditis, and rheumatic fever. Aspirin is also used long-term to help prevent further heart attacks, ischaemic strokes, and blood clots in people at high risk. For pain or fever, effects typically begin within 30 minutes. Aspirin works similarly to other NSAIDs but also suppresses the normal functioning of platelets. One common adverse effect is an upset stomach. More significant side effects include stomach ulcers, stomach bleeding, and worsening asthma. Bleeding risk is greater among those who are older, drink alcohol, take other NSAIDs, or are on other blood thinners. Aspirin is not recommended in the last part of pregnancy. It is not generally recommended in children with viral infections because of the risk of Reye syndrome. High doses may result in ringing in the ears. A precursor to aspirin found in the bark of the willow tree (genus Salix), salicin, is metabolized in the human gut into the medicinally active compound salicylic acid and has been used for its health effects for at least 2,400 years. Pharmacology sought a synthetic alternative. In 1853, the chemist Charles Frédéric Gerhardt treated the medicine sodium salicylate with acetyl chloride to produce acetylsalicylic acid for the first time. Over the next 50 years, other chemists, mostly of the German company Bayer, established the chemical structure and devised more efficient production methods. Felix Hoffmann (or perhaps Arthur Eichengrün) of Bayer was the first to produce acetylsalicylic acid in a pure, stable form in 1897. By 1899, Bayer had dubbed this drug Aspirin and was selling it globally. Aspirin is available without medical prescription as a proprietary or generic medication in most jurisdictions. It is one of the most widely used medications globally, with an estimated 40,000 tonnes (44,000 tons) (50 to 120 billion pills) consumed each year, and is on the World Health Organization's List of Essential Medicines. In 2023, it was the 46th most commonly prescribed medication in the United States, with more than 14 million prescriptions.

Human anatomy (gr. ἀνατομία, "dissection", from ἀνά, "up", and τέμνειν, "cut") is primarily the scientific study of the morphology of the human body. Anatomy is subdivided into gross anatomy and microscopic anatomy. Gross anatomy (also called macroscopic anatomy, topographical anatomy, regional anatomy, or anthropotomy) is the study of anatomical structures that can be seen by the naked eye. Microscopic anatomy is the study of minute anatomical structures assisted with microscopes, which includes histology (the study of the organization of tissues), and cytology (the study of cells). Anatomy, human physiology (the study of function), and biochemistry (the study of the chemistry of living structures) are complementary basic medical sciences that are generally together (or in tandem) to students studying medical sciences. In some of its facets human anatomy is closely related to embryology, comparative anatomy and comparative embryology, through common roots in evolution; for example, much of the human body maintains the ancient segmental pattern that is present in all vertebrates with basic units being repeated, which is particularly obvious in the vertebral column and in the ribcage, and can be traced from very early embryos. The human body consists of biological systems, that consist of organs, that consist of tissues, that consist of cells and connective tissue. The history of anatomy has been characterized, over a long period of time, by a continually developing understanding of the functions of organs and structures of the body. Methods have also advanced dramatically, advancing from examination of animals through dissection of fresh and preserved cadavers (corpses) to technologically complex techniques developed in the 20th century.

Mental health encompasses emotional, psychological, and social well-being, influencing cognition, perception, and behavior. Mental health plays a crucial role in an individual's daily life when managing stress, engaging with others, and contributing to life overall. According to the World Health Organization (WHO), it is a "state of well-being in which the individual realizes their abilities, can cope with the normal stresses of life, can work productively and fruitfully, and can contribute to their community". It likewise determines how an individual handles stress, interpersonal relationships, and decision-making. Mental health includes subjective well-being, perceived self-efficacy, autonomy, competence, intergenerational dependence, and self-actualization of one's intellectual and emotional potential, among others. Mental health also includes emotional, psychological, and social well-being, and it affects how people manage stress and make everyday decisions. From the perspectives of positive psychology or holism, mental health is thus not merely the absence of mental illness. Rather, it is a broader state of well-being that includes an individual's ability to enjoy life and to create a balance between life activities and efforts to achieve psychological resilience. Cultural differences, personal philosophy, subjective assessments, and competing professional theories all affect how one defines "mental health".

In multicellular organisms, stem cells are undifferentiated or partially differentiated cells that can change into various types of cells and proliferate indefinitely to produce more of the same stem cell. They are the earliest type of cell in a cell lineage. They are found in both embryonic and adult organisms, but they have slightly different properties in each. They are usually distinguished from progenitor cells, which cannot divide indefinitely, and precursor or blast cells, which are usually committed to differentiating into one cell type. In mammals, roughly 50 to 150 cells make up the inner cell mass during the blastocyst stage of embryonic development, around days 5–14. These have stem-cell capability. In vivo, they eventually differentiate into all of the body's cell types (making them pluripotent). This process starts with the differentiation into the three germ layers – the ectoderm, mesoderm and endoderm – at the gastrulation stage. However, when they are isolated and cultured in vitro, they can be kept in the stem-cell stage and are known as embryonic stem cells (ESCs). Adult stem cells are found in a few select locations in the body, known as niches, such as those in the bone marrow or gonads. They exist to replenish rapidly lost cell types and are multipotent or unipotent, meaning they only differentiate into a few cell types or one type of cell. In mammals, they include, among others, hematopoietic stem cells, which replenish blood and immune cells, basal cells, which maintain the skin epithelium, and mesenchymal stem cells, which maintain bone, cartilage, muscle and fat cells. Adult stem cells are a small minority of cells; they are vastly outnumbered by the progenitor cells and terminally differentiated cells that they differentiate into. Research into stem cells grew out of findings by Canadian biologists Ernest McCulloch, James Till and Andrew J. Becker at the University of Toronto and the Ontario Cancer Institute in the 1960s. As of 2016, the only established medical therapy using stem cells is hematopoietic stem cell transplantation, first performed in 1958 by French oncologist Georges Mathé. Since 1998 however, it has been possible to culture and differentiate human embryonic stem cells (in stem-cell lines). The process of isolating these cells has been controversial, because it typically results in the destruction of the embryo. Sources for isolating ESCs have been restricted in some European countries and Canada, but others such as the UK and China have promoted the research. Somatic cell nuclear transfer is a cloning method that can be used to create a cloned embryo for the use of its embryonic stem cells in stem cell therapy. In 2006, a Japanese team led by Shinya Yamanaka discovered a method to convert mature body cells back into stem cells. These were termed induced pluripotent stem cells (iPSCs).

A pandemic ( pan-DEM-ik) is an epidemic of an infectious disease that has a sudden increase in cases and spreads across a large region, for instance multiple continents or worldwide, affecting a substantial portion of the human population. Widespread endemic diseases with a stable number of infected individuals such as recurrences of seasonal influenza are generally excluded as they occur simultaneously in large regions of the globe rather than being spread worldwide. Throughout human history, there have been a number of pandemics of diseases such as smallpox. The Black Death, caused by the Plague, caused the deaths of up to half of the population of Europe in the 14th century. The term pandemic had not been used then, but was used for later epidemics, including the 1918 H1N1 influenza A pandemic—more commonly known as the Spanish flu—which is the deadliest pandemic in history. The most recent pandemics include the HIV/AIDS pandemic, the 2009 swine flu pandemic and the COVID-19 pandemic. Almost all these diseases still circulate among humans though their impact now is often far less. In response to the COVID-19 pandemic, 194 member states of the World Health Organization began negotiations on an International Treaty on Pandemic Prevention, Preparedness and Response, with a requirement to submit a draft of this treaty to the 77th World Health Assembly during its 2024 convention. The WHO Pandemic Agreement was adopted at the World Health Assembly in May 2025, establishing rules on how countries should act and collaborate to prevent, prepare for and respond to pandemic emergencies and public health emergencies of international concern. It was approved with 120 votes in favour, 70 abstentions and the absence of the United States, which had already withdrawn from the WHO.

A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Viruses are found in almost every ecosystem on Earth and are the most numerous type of biological entity. Since Dmitri Ivanovsky's 1892 article describing a non-bacterial pathogen infecting tobacco plants and the discovery of the tobacco mosaic virus by Martinus Beijerinck in 1898, more than 16,000 of the millions of virus species have been described in detail. The study of viruses is known as virology, a subspeciality of microbiology. When infected, a host cell is often forced to rapidly produce thousands of copies of the original virus. When not inside an infected cell or in the process of infecting a cell, viruses exist in the form of independent viral particles, or virions, consisting of (i) genetic material, i.e., long molecules of DNA or RNA that encode the structure of the proteins by which the virus acts; (ii) a protein coat, the capsid, which surrounds and protects the genetic material; and in some cases (iii) an outside envelope of lipids. The shapes of these virus particles range from simple helical and icosahedral forms to more complex structures. Most virus species have virions too small to be seen with an optical microscope and are one-hundredth the size of most bacteria. The origins of viruses in the evolutionary history of life are still unclear. Some viruses may have evolved from plasmids, which are pieces of DNA that can move between cells. Other viruses may have evolved from bacteria. In evolution, viruses are an important means of horizontal gene transfer, which increases genetic diversity in a way analogous to sexual reproduction. Viruses are considered by some biologists to be a life form, because they carry genetic material, reproduce, and evolve through natural selection, although they lack some key characteristics, such as cell structure, that are generally considered necessary criteria for defining life. Because they possess some but not all such qualities, viruses have been described as "organisms at the edge of life" and as replicators. Viruses spread in many ways. One transmission pathway is through disease-bearing organisms known as vectors: for example, viruses are often transmitted from plant to plant by insects that feed on plant sap, such as aphids; and viruses in animals can be carried by blood-sucking insects. Many viruses spread in the air by coughing and sneezing, including influenza viruses, SARS-CoV-2, chickenpox, smallpox, and measles. Norovirus and rotavirus, common causes of viral gastroenteritis, are transmitted by the faecal–oral route, passed by hand-to-mouth contact or in food or water. The infectious dose of norovirus required to produce infection in humans is fewer than 100 particles. HIV is one of several viruses transmitted through sexual contact and by exposure to infected blood. The variety of host cells that a virus can infect is called its host range: this is narrow for viruses specialized to infect only a few species, or broad for viruses capable of infecting many. Viral infections in animals provoke an immune response that usually eliminates the infecting virus. Immune responses can also be produced by vaccines, which confer an artificially acquired immunity to the specific viral infection. Some viruses, including those that cause HIV/AIDS, HPV infection, and viral hepatitis, evade these immune responses and result in chronic infections. Several classes of antiviral drugs have been developed.

An antibiotic is a type of antimicrobial substance active against bacteria. It is the most important type of antibacterial agent for fighting bacterial infections, and antibiotic medications are widely used in the treatment and prevention of such infections. They may either kill or inhibit the growth of bacteria. A limited number of antibiotics also possess antiprotozoal activity. Antibiotics are not effective against viruses such as the ones which cause the common cold or influenza. Drugs which inhibit growth of viruses are termed antiviral drugs or antivirals. Antibiotics are also not effective against fungi. Drugs which inhibit growth of fungi are called antifungal drugs. Sometimes, the term antibiotic—literally "opposing life", from the Greek roots ἀντι anti, "against" and βίος bios, "life"—is broadly used to refer to any substance used against microbes, but in the usual medical usage, antibiotics (such as penicillin) are those produced naturally (by one microorganism fighting another), whereas non-antibiotic antibacterials (such as sulfonamides and antiseptics) are fully synthetic. However, both classes have the same effect of killing or preventing the growth of microorganisms, and both are included in antimicrobial chemotherapy. "Antibacterials" include bactericides, bacteriostatics, antibacterial soaps, and chemical disinfectants, whereas antibiotics are an important class of antibacterials used more specifically in medicine and sometimes in livestock feed.

The germ theory of disease is the currently accepted scientific theory explaining the cause of infectious diseases. It states that microorganisms known as pathogens or "germs" can cause disease. These small organisms, which are too small to be seen without magnification, invade animals, plants, and even bacteria. Their growth and reproduction within their hosts can cause disease. "Germ" refers not just to bacteria but to any type of microorganism, such as protists or fungi, or other pathogens, including parasites, viruses, prions, or viroids. Even when a pathogen is the principal cause of a disease, environmental and hereditary factors often influence the severity of the disease, and whether a potential host individual becomes infected when exposed to the pathogen. Pathogens are disease-causing agents that can pass from one individual to another, across multiple domains of life. Basic forms of germ theory were proposed by Girolamo Fracastoro in 1546, and expanded upon by Marcus von Plenciz in 1762. However, such views were held in disdain in Europe, where Galen's miasma theory remained dominant among scientists and doctors. By the early 19th century, the first vaccine, smallpox vaccination, was commonplace in Europe, though doctors were unaware of how it worked or how to extend the principle to other diseases. A transitional period began in the late 1850s with the work of Louis Pasteur. This work was later extended by Robert Koch in the 1880s. By the end of that decade, the miasma theory was struggling to compete with the germ theory of disease. Viruses were initially discovered in the 1890s. Eventually, a "golden era" of bacteriology ensued, during which the germ theory quickly led to the identification of the actual organisms that cause many diseases.

Cardiology (from Ancient Greek καρδίᾱ (kardiā) 'heart' and -λογία (-logia) 'study') is the study of the heart. Cardiology is a branch of medicine that deals with disorders of the heart and the cardiovascular system, and it is a sub-specialty of internal medicine. The field includes medical diagnosis and treatment of congenital heart defects, coronary artery disease, heart failure, valvular heart disease, and electrophysiology. Physicians who specialize in this field of medicine are called cardiologists. Pediatric cardiologists are pediatricians who specialize in cardiology. Physicians who specialize in cardiac surgery are called cardiothoracic surgeons or cardiac surgeons, a specialty of general surgery.

Neurology (from Greek: νεῦρον (neûron), "string, nerve" and the suffix -logia, "study of") is the branch of medicine dealing with the diagnosis and treatment of all categories of conditions and disease involving the nervous system, which comprises the brain, the spinal cord and the peripheral nerves. Neurological practice relies heavily on the field of neuroscience, the scientific study of the nervous system, using various techniques of neurotherapy. A neurologist is a physician specializing in neurology and trained to investigate, diagnose and treat neurological disorders. Neurologists diagnose and treat myriad neurologic conditions, including stroke, epilepsy, movement disorders such as Parkinson's disease, brain infections, autoimmune neurologic disorders such as multiple sclerosis, sleep disorders, brain injury, headache disorders like migraine, tumors of the brain and dementias such as Alzheimer's disease. Neurologists may also have roles in clinical research, clinical trials, and basic or translational research. Neurology is a nonsurgical specialty; its corresponding surgical specialty is neurosurgery.

Psychiatry is the medical specialty devoted to the diagnosis, treatment, and prevention of deleterious mental conditions. These include matters related to cognition, perceptions, mood, emotion, and behavior. Initial psychiatric assessment begins with taking a case history and conducting a mental status examination. Laboratory tests, physical examinations, and psychological assessments may also be used. On occasion, neuroimaging or neurophysiological studies are performed. Mental disorders are diagnosed in accordance with diagnostic manuals such as the International Classification of Diseases (ICD), edited by the World Health Organization (WHO), and the Diagnostic and Statistical Manual of Mental Disorders (DSM), published by the American Psychiatric Association (APA). The fifth edition of the DSM (DSM-5) was published in May 2013. However, its release coincided with a statement by then-NIMH director Thomas Insel noting that the manual lacks validity compared to other medical specialties due to the absence of objective laboratory measures (biomarkers) for diagnosis. Treatment may include psychotropics (psychiatric medicines), psychotherapy, substance-abuse treatment, and other modalities such as interventional approaches, assertive community treatment, community reinforcement, and supported employment. Treatment may be delivered on an inpatient or outpatient basis, depending on the severity of functional impairment or risk to the individual or community. Research within psychiatry is conducted by psychiatrists on an interdisciplinary basis with other professionals, including clinical psychologists, epidemiologists, nurses, social workers, and occupational therapists. Psychiatry has been controversial since its inception, facing criticism both internally and externally over its medicalization of mental distress, reliance on pharmaceuticals, use of coercion, influence from the pharmaceutical industry, and its historical role in social control and contentious treatments.

Epidemiology is the study and analysis of the distribution (who, when, and where), patterns and determinants of health and disease conditions in a defined population, and application of this knowledge to prevent diseases. It is a cornerstone of public health, and shapes policy decisions and evidence-based practice by identifying risk factors for disease and targets for preventive healthcare. Epidemiologists help with study design, collection, and statistical analysis of data, amend interpretation and dissemination of results (including peer review and occasional systematic review). Epidemiology has helped develop methodology used in clinical research, public health studies, and, to a lesser extent, basic research in the biological sciences. Major areas of epidemiological study include disease causation, transmission, outbreak investigation, disease surveillance, environmental epidemiology, forensic epidemiology, occupational epidemiology, screening, biomonitoring, and comparisons of treatment effects such as in clinical trials. Epidemiologists rely on other scientific disciplines like biology to better understand disease processes, statistics to make efficient use of the data and draw appropriate conclusions, social sciences to better understand proximate and distal causes, and engineering for exposure assessment. Epidemiology, literally meaning 'the study of what is upon the people', is derived from Greek epi 'upon, among'; demos 'people' and logos 'study', referring to the study of human populations. However, the term is widely used in studies of zoological populations (veterinary epidemiology), although the term 'epizootiology' was historically used. 'Epidemiology' has also been applied to studies of plant populations (botanical or plant disease epidemiology). The distinction between "epidemic" and "endemic" was first drawn by Hippocrates, The term "epidemiology" appears to have first been used to describe the study of epidemics in 1802 by the Spanish physician Joaquín de Villalba in Epidemiología Española. Epidemiologists also study the interaction of diseases in a population, a condition known as a syndemic. The term epidemiology is now widely applied to cover the description and causation of not only epidemic, infectious disease, but of disease in general, including related conditions and, especially since the 20th century, chronic diseases such as diabetes, cardiovascular disease, and cancer. Some examples of topics examined through epidemiology include high blood pressure, mental illness and obesity. Therefore, this epidemiology is based upon how the pattern of the disease causes change in the function of human beings.

The Lion King is a 1994 American animated musical drama film directed by Roger Allers and Rob Minkoff, produced by Don Hahn, and written by Irene Mecchi, Jonathan Roberts, and Linda Woolverton. It was produced by Walt Disney Feature Animation, and features an ensemble voice cast consisting of Matthew Broderick, James Earl Jones, Jeremy Irons, Jonathan Taylor Thomas, Moira Kelly, Niketa Calame, Nathan Lane, Ernie Sabella, Whoopi Goldberg, Cheech Marin, Rowan Atkinson, and Robert Guillaume. The story follows a young lion prince named Simba, who is exiled from his kingdom after his father Mufasa is murdered by his uncle Scar to seize the throne. As he grows up, Simba must decide whether to return home to confront Scar and reclaim his place as king. The Lion King was conceived during conversations among various Disney executives, to whom several writers submitted early treatments under the title King of the Jungle. Original director George Scribner had envisioned The Lion King as a nature documentary-style film, with Allers joining as co-director after having worked in the story departments of several successful animated Disney films. The Lion King's plot draws inspiration from several sources, notably William Shakespeare's play Hamlet. Woolverton, screenwriter for Disney's Beauty and the Beast (1991), drafted early versions of The Lion King's script, which Mecchi and Roberts were hired to revise once Woolverton left to prioritize other projects. Scribner departed due to disagreements over the studio's decision to reimagine the film as a musical, with original songs by Elton John and Tim Rice, and Minkoff was hired to replace him in April 1992. Throughout production, the creative team visited Kenya for research and inspiration. Released in theaters on June 24, 1994 by Walt Disney Pictures, The Lion King was praised by critics for its music, story, themes, and animation. With an initial worldwide gross of $763 million, it completed its theatrical run as the highest-grossing film of 1994 and the second-highest-grossing film of all time, behind Jurassic Park (1993). It held the title of highest-grossing animated film until it was replaced by Finding Nemo in 2003. The film remains the highest-grossing traditionally animated film of all time, having its initial gross remained the highest-grossing initial gross for a traditionally animated film until it was surpassed 31 years later by Demon Slayer: Kimetsu no Yaiba – The Movie: Infinity Castle in 2025. The film was also the best-selling film on home video, having sold over 55 million copies worldwide. It won two Academy Awards, as well as the Golden Globe Award for Best Motion Picture – Musical or Comedy. It is considered by many to be among the greatest animated films ever made. The success of the film launched a multibillion-dollar franchise comprising a Broadway adaptation, two direct-to-video follow-ups, two television series, and a photorealistic remake (which itself spawned a prequel), which in 2019 also became the highest-grossing animated film at the time of its release. In 2016, The Lion King was selected for preservation in the United States National Film Registry by the Library of Congress as being "culturally, historically, or aesthetically significant".

The Silence of the Lambs is a 1991 American psychological horror thriller film directed by Jonathan Demme and written by Ted Tally, adapted from Thomas Harris's 1988 novel. It stars Jodie Foster as Clarice Starling, a young FBI trainee who is hunting a serial killer known as "Buffalo Bill" (Ted Levine), who skins his female victims. To catch him, she seeks the advice of the imprisoned Hannibal Lecter (Anthony Hopkins), a brilliant psychiatrist and cannibalistic serial murderer. The film also features performances by Scott Glenn, Anthony Heald, and Kasi Lemmons. Released by Orion Pictures on February 14, 1991, the film grossed $272.7 million worldwide on a $19 million budget and became the fifth-highest-grossing film of the year. It premiered at the 41st Berlin International Film Festival, where Demme won the Silver Bear for Best Director. At the 64th Academy Awards, it became the third—and most recent—film to win the "Big Five" categories: Best Picture, Best Director, Best Actor, Best Actress, and Best Adapted Screenplay. It remains the only horror film to have won Best Picture. The Silence of the Lambs is regularly cited by critics, film directors, and audiences as one of the greatest and most influential films. In 2018, Empire ranked it 48th on its list of the 500 greatest movies of all time. The American Film Institute ranked it the sixty-fifth greatest film in American cinema and the fifth-greatest thriller, while Starling and Lecter were included among the greatest film heroines and villains. The film was deemed "culturally, historically, or aesthetically" significant by the U.S. Library of Congress and was selected for preservation in the National Film Registry in 2011. The film has drawn criticism for its portrayal of gender identity, particularly regarding the character of Buffalo Bill, which some critics argue perpetuates harmful stereotypes. Demme defended the film's intentions, arguing that it merely tells the personal story of "a tormented man". The film launched a franchise that includes the sequel Hannibal (2001), the prequels Red Dragon (2002) and Hannibal Rising (2007), and two television adaptations.

Avengers: Endgame is a 2019 American superhero film based on the Marvel Comics superhero team the Avengers. Produced by Marvel Studios and distributed by Walt Disney Studios Motion Pictures, it is the direct sequel to Avengers: Infinity War (2018) and the 22nd film in the Marvel Cinematic Universe (MCU). Directed by Anthony and Joe Russo and written by Christopher Markus and Stephen McFeely, the film features an ensemble cast that includes Robert Downey Jr., Chris Evans, Mark Ruffalo, Chris Hemsworth, Scarlett Johansson, Jeremy Renner, Don Cheadle, Paul Rudd, Brie Larson, Karen Gillan, Danai Gurira, Benedict Wong, Jon Favreau, Bradley Cooper, Gwyneth Paltrow, and Josh Brolin. In the film, the surviving members of the Avengers and their allies attempt to reverse Thanos's actions in Infinity War, which erased half of all life in the universe. The film was announced in October 2014 as Avengers: Infinity War – Part 2, but Marvel Studios later removed this title. The Russo brothers joined as directors in April 2015, with Markus and McFeely signing on to write the script a month later. It is a conclusion to the story of the MCU up to that point, ending several main characters' story arcs. The plot revisits moments from earlier films, bringing back actors and settings from throughout the franchise. Filming began in August 2017 at Pinewood Atlanta Studios in Fayette County, Georgia, shooting back-to-back with Infinity War, and ended in January 2018. Additional filming took place in the Metro and downtown Atlanta areas, New York state, Scotland, and England. The official title was announced in December 2018. With an estimated budget range of $356–400 million, Endgame is one of the most expensive films ever produced. Avengers: Endgame premiered at the Los Angeles Convention Center on April 22, 2019, and was released in the United States on April 26 as part of Phase Three of the MCU. The film received positive reviews from critics, with praise for its direction, acting, musical score, action scenes, visual effects, and emotional weight, with critics lauding its culmination of the 22-film story. It grossed nearly $2.8 billion worldwide, surpassing Infinity War's entire theatrical run in eleven days and setting several box-office records. It was the highest-grossing film of all time from July 2019 to March 2021, when it was surpassed by the previous record-holder, Avatar (2009). Endgame was nominated for Best Visual Effects at the 92nd Academy Awards, among numerous other accolades. Two sequels, Avengers: Doomsday and Avengers: Secret Wars, are scheduled for release in 2026 and 2027.

Interstellar is a 2014 epic science fiction film directed by Christopher Nolan, who co-wrote the screenplay with his brother Jonathan Nolan. It has an ensemble cast led by Matthew McConaughey, Anne Hathaway, Jessica Chastain, Bill Irwin, Ellen Burstyn, and Michael Caine. Set in a dystopian future where Earth is suffering from catastrophic blight and famine, the film follows a group of astronauts who travel through space in search of a new home for humanity. The screenplay had its origins in a script that Jonathan had developed in 2007 and was originally set to be directed by Steven Spielberg. Theoretical physicist Kip Thorne was an executive producer and scientific consultant on the film, and wrote the tie-in book The Science of Interstellar. It was Lynda Obst's final film as producer before her death. Cinematographer Hoyte van Hoytema shot it on 35 mm film in the Panavision anamorphic format and IMAX 70 mm. Filming began in late 2013 and took place in Alberta, Klaustur, and Los Angeles. Interstellar uses extensive practical and miniature effects, and the company DNEG created additional visual effects. Interstellar premiered at the TCL Chinese Theatre on October 26, 2014, and was released in theaters in the United States on November 5, and in the United Kingdom on November 7, with Paramount Pictures distributing in the United States and Warner Bros. Pictures distributing in international markets. In the United States, it was first released on film stock, expanding to venues using digital projectors. It was a commercial success, grossing $681 million worldwide during its initial theatrical run, and $774.7 million worldwide with subsequent releases, making it the 10th-highest-grossing film of 2014. The film received generally positive reviews from critics. Among its various accolades, Interstellar was nominated for five awards at the 87th Academy Awards, winning Best Visual Effects.

Psycho is a 1960 American horror film produced and directed by Alfred Hitchcock. The screenplay, written by Joseph Stefano, is based the 1959 novel by Robert Bloch. The film stars Anthony Perkins, Janet Leigh, Vera Miles, John Gavin and Martin Balsam. The plot centers on an encounter between on-the-run embezzler Marion Crane (Leigh), shy motel proprietor Norman Bates (Perkins) and his disturbed mother. A private investigator (Balsam), Marion's lover Sam Loomis (Gavin) and her sister Lila (Miles) investigate her disappearance. Psycho was seen as a departure from Hitchcock's previous film, North by Northwest (1959), as it was filmed on a small budget in black-and-white by the crew of his television series Alfred Hitchcock Presents. Initially, the film divided critics due to its controversial subject matter, but audience interest and outstanding box-office returns prompted a major critical re-evaluation. Psycho was a massive commercial success; from a budget of US$806,947 (equivalent to $8.8 million in 2025), the film gained $50 million (equivalent to $544 million in 2025) at the box office, worldwide, not including the money made with rentals. Psycho was nominated for four Academy Awards, including Best Director for Hitchcock and Best Supporting Actress for Leigh. Psycho is now considered one of Hitchcock's best films, and is arguably his most famous and influential work. It has been hailed as a major work of cinematic art by international film critics and scholars who praise its slick direction, tense atmosphere, impressive camerawork, memorable score and iconic performances. Additionally, the shower scene has become one of the best-known in all of cinema. It is regarded as "the most heavily analyzed film in the long career of the most investigated director in the history of American film" and often ranked among the greatest films of all time. It set a new level of acceptability for violence, deviant behavior and sexuality in American films, and has been considered to be one of the earliest examples of the slasher film genre. After Hitchcock's death in 1980, Universal Pictures produced follow-ups: three sequels, a remake, a made-for-television spin-off and a television series. In 1992, the Library of Congress deemed the film "culturally, historically, or aesthetically significant" and selected it for preservation in the United States National Film Registry.

Goodfellas (stylized as GoodFellas) is a 1990 American biographical gangster film directed by Martin Scorsese, written by Nicholas Pileggi and Scorsese, and produced by Irwin Winkler. It is a film adaptation of Pileggi's 1985 nonfiction book Wiseguy. Starring Robert De Niro, Ray Liotta, Joe Pesci, Lorraine Bracco and Paul Sorvino, the film narrates the rise and fall of Mafia associate Henry Hill and his friends and family from 1955 to 1980. Scorsese initially titled the film Wise Guy and postponed making it; he and Pileggi later changed the title to Goodfellas. To prepare for their roles in the film, De Niro, Pesci and Liotta often spoke with Pileggi, who shared research material remaining from writing the book. According to Pesci, improvisation and ad-libbing came from rehearsals wherein Scorsese gave the actors freedom to do whatever they wanted. The director made transcripts of these sessions, took the lines that he liked most and put them into a revised script, from which the cast worked during principal photography. Goodfellas premiered at the 47th Venice International Film Festival on September 9, 1990, where Scorsese was awarded with the Silver Lion award for Best Director, and was released in the United States on September 18 by Warner Bros.. When released, the film grossed $47 million against a budget of $25 million and received widespread acclaim. The film was nominated for six Academy Awards, including Best Picture and Best Director, with Pesci winning Best Supporting Actor. The film also won five awards from the British Academy of Film and Television Arts, including Best Film and Best Director, and was named the year's best film by various critics' groups. Widely regarded as one of the greatest films ever made, particularly in the gangster genre, the critical consensus on review aggregator Rotten Tomatoes calls it "arguably the high point of Martin Scorsese's career". In 2000, it was deemed "culturally, historically, or aesthetically significant" and selected for preservation in the National Film Registry by the United States Library of Congress. Its content and style have been emulated in numerous other pieces of media.

Gladiator is a 2000 epic action adventure drama film directed by Ridley Scott and written by David Franzoni, John Logan, and William Nicholson from a story by Franzoni. It stars Russell Crowe, Joaquin Phoenix, Connie Nielsen, Oliver Reed, Derek Jacobi, Djimon Hounsou, and Richard Harris. Crowe portrays Maximus Decimus Meridius, a Roman general who is betrayed when Commodus, the ambitious son of Emperor Marcus Aurelius, murders his father and seizes the throne. Reduced to a slave, Maximus becomes a gladiator and rises through the ranks of the arena, determined to avenge the murders of his family and the emperor. The screenplay, initially written by Franzoni, was inspired by the 1958 Daniel P. Mannix novel Those About to Die. The script was acquired by DreamWorks Pictures, and Scott signed on to direct the film. Principal photography began in January 1999 and wrapped in May of that year. Production was complicated by the script being rewritten multiple times and by the death of Oliver Reed before production was finished. Gladiator had its world premiere in Los Angeles, California, on May 1, 2000. The film was released in the United States on May 5, 2000 by DreamWorks Distribution LLC, and internationally on May 12, 2000, by Universal Pictures through United International Pictures. The film grossed $466 million worldwide, becoming the second-highest-grossing film of 2000, and won five Academy Awards, including Best Picture and Best Actor for Crowe. A sequel, Gladiator II, was released in 2024.

Back to the Future is a 1985 American science fiction film directed by Robert Zemeckis and written by Zemeckis and Bob Gale. It stars Michael J. Fox, Christopher Lloyd, Lea Thompson, Crispin Glover, and Thomas F. Wilson. Set in 1985, it follows Marty McFly (Fox), a teenager accidentally sent back to 1955 in a time-traveling DeLorean automobile built by his eccentric scientist friend Emmett "Doc" Brown (Lloyd), where he inadvertently prevents his future parents from falling in love – threatening his own existence – and is forced to reconcile them and somehow get back to the future. Gale and Zemeckis conceived the idea for Back to the Future in 1980. They were desperate for a successful film after numerous collaborative failures, but the project was rejected more than forty times by various studios because it was not considered raunchy enough to compete with the successful comedies of the era. A development deal was secured with Universal Pictures following Zemeckis's success directing Romancing the Stone (1984). Fox was the first choice to portray Marty but was unavailable; Eric Stoltz was cast instead. Shortly after principal photography began in November 1984, Zemeckis determined Stoltz was not right for the part and made the concessions necessary to hire Fox, including re-filming scenes already shot with Stoltz and adding $4 million to the budget. Back to the Future was filmed in and around California and on sets at Universal Studios, and concluded the following April. After highly successful test screenings, the release date was brought forward to July 3, 1985, giving the film more time in theaters during the busiest period of the theatrical year. The change resulted in a rushed post-production schedule and some incomplete special effects. Nevertheless, Back to the Future was a critical and commercial success, earning $381.1 million to become the highest-grossing film of 1985 worldwide. Critics praised the story, humor, and the cast, particularly Fox, Lloyd, Thompson, and Glover. It received multiple award nominations and won an Academy Award, three Saturn Awards, and a Hugo Award. Its theme song, "The Power of Love" by Huey Lewis and the News, was also a success. Back to the Future has since grown in esteem and is now considered by critics and audiences to be one of the greatest science fiction films and among the best films ever made. In 2007, the United States Library of Congress selected it for preservation in the National Film Registry. The film was followed by two sequels, Back to the Future Part II (1989) and Back to the Future Part III (1990). Spurred by the film's dedicated fan following and effect on popular culture, Universal Studios launched a multimedia franchise, which now includes video games, theme park rides, an animated television series, and a stage musical. Its enduring popularity has prompted numerous books about its production, documentaries, and commercials.

Inception is a 2010 science fiction action film written and directed by Christopher Nolan, who also produced it with Emma Thomas, his wife. The film stars Leonardo DiCaprio as a professional thief who steals information by infiltrating the subconscious of his targets. He is offered a chance to have his criminal history erased as payment for the implantation of another person's idea into a target's subconscious. The ensemble cast includes Ken Watanabe, Joseph Gordon-Levitt, Marion Cotillard, Elliot Page, Tom Hardy, Cillian Murphy, Tom Berenger, Dileep Rao, and Michael Caine. After the completion of Insomnia in 2002, Nolan presented to Warner Bros. a written 80-page treatment for a horror film envisioning "dream stealers," based on lucid dreaming. Deciding he needed more experience before tackling a production of this magnitude and complexity, Nolan shelved the project and instead worked on Batman Begins (2005), The Prestige (2006), and The Dark Knight (2008). The treatment was revised over six months and was purchased by Warner Bros. in February 2009. Inception was filmed in six countries, beginning in Tokyo on June 19 and ending in Canada on November 22. Its official budget was $160 million, split between Warner Bros. and Legendary. Nolan's reputation and success with The Dark Knight helped secure the film's US$100 million in advertising expenditure. Inception's premiere was held in London on July 8, 2010; it was released in both conventional and IMAX theaters beginning on July 16, 2010. Inception grossed $839 million worldwide, becoming the fourth-highest-grossing film of 2010. Considered one of the best films of the 2010s, Inception, among its numerous accolades, won four Oscars (Best Cinematography, Best Sound Editing, Best Sound Mixing, Best Visual Effects) and was nominated for four more (Best Picture, Best Original Screenplay, Best Art Direction, Best Original Score) at the 83rd Academy Awards. In 2025, the Library of Congress selected Inception for preservation in the United States National Film Registry for being "culturally, historically or aesthetically significant".

E.T. the Extra-Terrestrial (or simply E.T.) is a 1982 American science fantasy film produced and directed by Steven Spielberg and written by Melissa Mathison. It tells the story of Elliott, a boy who befriends an extraterrestrial that he names E.T. who has been stranded on Earth. Along with his friends and family, Elliott must find a way to help E.T. find his way home. The film stars Dee Wallace, Henry Thomas, Peter Coyote, Robert MacNaughton, and Drew Barrymore. The film's concept was based on an imaginary friend that Spielberg created after his parents' divorce. In 1980, Spielberg met Mathison and developed a new story from the unrealized project Night Skies. In less than two months, Mathison wrote the first draft of the script, titled E.T. and Me, which went through two rewrites. The project was rejected by Columbia Pictures, who doubted its commercial potential. Universal Pictures eventually purchased the script for $1 million. Filming took place from September to December 1981 on a budget of $10.5 million. Unlike most films, E.T. was shot in rough chronological order to facilitate convincing emotional performances from the young cast. The animatronics for the film were designed by Carlo Rambaldi. E.T. premiered as the closing film of the Cannes Film Festival on May 26, 1982, and was released in the United States on June 11. The film was a smash hit at the box office, surpassing Star Wars (1977) to become the highest-grossing film of all time, a record it held for eleven years until Spielberg's own Jurassic Park surpassed it in 1993. E.T. received universal acclaim from critics, and is regarded as one of the greatest and most influential films ever made. It received nine nominations at the 55th Academy Awards, winning Best Original Score, Best Visual Effects, Best Sound, and Best Sound Editing in addition to being nominated for Best Picture and Best Director. It also won five Saturn Awards and two Golden Globe Awards. The film was re-released in 1985 and again in 2002 to celebrate its 20th anniversary, with altered shots, visual effects, and additional scenes. It was also re-released in IMAX on August 12, 2022, to celebrate its 40th anniversary. In 1994, the film was added to the United States National Film Registry of the Library of Congress, who deemed it "culturally, historically, or aesthetically significant."

Schindler's List is a 1993 American epic historical drama film directed and co-produced by Steven Spielberg and written by Steven Zaillian. It is based on the historical novel Schindler's Ark (1982) by Thomas Keneally. The film follows Oskar Schindler, a German industrialist who saved more than a thousand mostly Polish–Jewish refugees from the Holocaust by employing them in his factories during World War II. It stars Liam Neeson as Schindler, Ralph Fiennes as SS officer Amon Göth, and Ben Kingsley as Schindler's Jewish accountant Itzhak Stern. Ideas for a film about the Schindlerjuden (Schindler Jews) were proposed as early as 1963. Poldek Pfefferberg, one of the Schindlerjuden, made it his life's mission to tell Schindler's story. Spielberg became interested when executive Sidney Sheinberg sent him a book review of Schindler's Ark. Universal Pictures bought the rights to the novel, but Spielberg, unsure if he was ready to make a film about the Holocaust, tried to pass the project to several directors before deciding to direct it himself. Principal photography took place in Kraków, Poland, over 72 days in 1993. Spielberg shot the film in black and white and approached the film as a documentary. Cinematographer Janusz Kamiński wanted to create a sense of timelessness. John Williams composed the score, and violinist Itzhak Perlman performed the main theme. Schindler's List premiered on November 30, 1993, in Washington, D.C., and was released on December 15, 1993, in the United States. The film received widespread critical acclaim, particularly for the performances of Neeson and Fiennes, Williams' musical score, Kaminski's cinematography, Zaillian's screenplay, and Spielberg's direction. It was a box office success, earning $322.2 million worldwide on a $22 million budget. Schindler's List earned multiple accolades, including seven Academy Awards (including Best Picture), seven BAFTAs and three Golden Globes. It is often listed as one of the greatest films ever made. The film was deemed "culturally, historically or aesthetically significant" by the Library of Congress in 2004 and selected for preservation in the U.S. National Film Registry. In 2007, the American Film Institute ranked Schindler's List 8th on its list of the 100 greatest American films.

Forrest Gump is a 1994 American comedy-drama film directed by Robert Zemeckis. An adaptation of the 1986 novel by Winston Groom, the film's screenplay was written by Eric Roth. It stars Tom Hanks in the title role, alongside Robin Wright, Gary Sinise, Mykelti Williamson, and Sally Field in lead roles. The film follows the life of an Alabama man named Forrest Gump (Hanks) and his experiences in the 20th-century United States. Principal photography took place between August and December 1993, mainly in Georgia, North Carolina, and South Carolina. Extensive visual effects were used to incorporate Hanks into archived footage and to develop other scenes. The soundtrack features songs reflecting the different periods seen in the film. Various interpretations have been made of the protagonist and the film's political symbolism. Forrest Gump was released in the United States by Paramount Pictures on July 6, 1994, and received widespread critical acclaim for Zemeckis's direction, the performances (particularly those of Hanks and Sinise), story, writing, emotional weight, visual effects, music, character development and screenplay. The film was a major success at the box office: it became the top-grossing film in the United States released that year and earned over US$678 million worldwide against a $55 million budget during its theatrical run, making it the second-highest-grossing film of 1994, behind The Lion King. The soundtrack sold over 12 million copies. Forrest Gump is often regarded as one of the greatest and most influential films ever made, and won six Academy Awards: Best Picture, Best Director, Best Actor for Hanks, Best Adapted Screenplay, Best Visual Effects, and Best Film Editing. It received many award nominations, including Golden Globes, British Academy Film Awards, and Screen Actors Guild Awards. In 2011, the Library of Congress selected the film for preservation in the United States National Film Registry as being "culturally, historically, or aesthetically significant".

Jurassic Park is a 1993 American science fiction adventure film directed by Steven Spielberg and written by Michael Crichton and David Koepp, based on Crichton's 1990 novel. Starring Sam Neill, Laura Dern, Jeff Goldblum, and Richard Attenborough, the film is set on the fictional island of Isla Nublar near Costa Rica, where wealthy businessman John Hammond (Attenborough) and a team of genetic scientists have created a wildlife park of de-extinct dinosaurs. When industrial sabotage leads to a catastrophic shutdown of the park's power facilities and security precautions, a small group of visitors struggle to survive and escape the now perilous island. Before Crichton's novel was published, four studios put in bids for its film rights. With the backing of Universal Pictures, Spielberg acquired the rights for $1.5 million. Crichton was hired for an additional $500,000 to adapt the novel for the screen. Koepp wrote the final draft, which left out much of the novel's exposition and violence, and featured numerous changes to the characters. Filming took place in California and Hawaii from August to November 1992. Post-production lasted until May 1993, supervised by Spielberg in Poland as he filmed Schindler's List. The dinosaurs were created with groundbreaking computer-generated imagery by Industrial Light & Magic, and with life-sized animatronic dinosaurs built by Stan Winston's team. To showcase the film's sound design, which included a mixture of various animal noises for the dinosaur sounds, Spielberg invested in the creation of DTS, a company specializing in digital surround sound formats. The film was backed by an extensive $65 million marketing campaign, which included licensing deals with over 100 companies. Jurassic Park premiered on June 9, 1993, at the Uptown Theater in Washington, D.C., and was released two days later throughout the United States. It was a blockbuster hit and went on to gross over $914 million worldwide in its original theatrical run, surpassing Spielberg's own E.T. the Extra-Terrestrial to become the highest-grossing film of all time until the release of Titanic (1997). The film received critical acclaim, with praise to its special effects, sound design, action sequences, John Williams's score, and Spielberg's direction. The film won 20 awards, including three Academy Awards for technical achievements in visual effects and sound design. Following its 20th anniversary re-release in 2013, Jurassic Park became the oldest film in history to surpass $1 billion in ticket sales and the 17th overall. Since its release, many film critics and industry professionals have regarded Jurassic Park as one of the greatest films ever made. Its groundbreaking use of computer-generated imagery is widely seen as a turning point that shaped the visual effects techniques used in modern cinema. In 2018, it was selected for preservation in the United States National Film Registry by the Library of Congress as "culturally, historically, or aesthetically significant". The film spawned a multimedia franchise that includes six sequels, video games, theme park attractions, comic books, and various merchandise.

Citizen Kane is a 1941 American drama film directed, produced by, and starring Orson Welles and co-written by Welles and Herman J. Mankiewicz. It was Welles's first feature film. The quasi-biographical film examines the life and legacy of Charles Foster Kane, played by Welles, a composite character based on American media barons William Randolph Hearst and Joseph Pulitzer, Chicago tycoons Samuel Insull and Harold McCormick, as well as aspects of the screenwriters' own lives. After the Broadway success of Welles's Mercury Theatre and the controversial 1938 radio broadcast "The War of the Worlds" on The Mercury Theatre on the Air, Welles was courted by Hollywood. He signed a contract with RKO Pictures in 1939. Although it was unusual for an untried director, he was given freedom to develop his own story, to use his own cast and crew, and to have final cut privilege. Following two abortive attempts to get a project off the ground, he wrote the screenplay for Citizen Kane with Herman J. Mankiewicz. Principal photography took place in 1940, the same year that its innovative trailer was shown, and the film was released in 1941. Upon its release, Hearst prohibited any mention of the film in his newspapers. Although it was a critical success, Citizen Kane failed to recoup its costs at the box office. The film faded from view after its release, but returned to public attention when it was praised by French critics such as André Bazin and re-released in 1956. In 1958, the film was voted number nine on the prestigious Brussels 12 list at the 1958 World Expo. Citizen Kane is frequently cited as the greatest film ever made. For 40 years (five decennial polls: 1962, 1972, 1982, 1992 and 2002), it stood at number one in the British Film Institute's Sight and Sound decennial poll, and it topped the American Film Institute's 100 Years ... 100 Movies list in 1998, as well as its 2007 update. The Library of Congress selected Citizen Kane as an inductee of the 1989 inaugural group of 25 films for preservation in the United States National Film Registry for being "culturally, historically, or aesthetically significant". The film was nominated for Academy Awards in nine categories and it won for Best Writing (Original Screenplay) by Mankiewicz and Welles. Citizen Kane is praised for Gregg Toland's cinematography, Robert Wise's editing, Bernard Herrmann's score and its narrative structure, all of which have been considered innovative and precedent-setting.

Parasite (Korean: 기생충) is a 2019 South Korean black comedy thriller film directed by Bong Joon Ho, who co-wrote the screenplay with Han Jin-won. It stars Song Kang-ho, Lee Sun-kyun, Cho Yeo-jeong, Choi Woo-shik, Park So-dam, Jang Hye-jin, Park Myung-hoon, and Lee Jung-eun. The film follows a poor family who infiltrate the home and life of a wealthy family. The script is based on a play Bong wrote in 2013. He later adapted it into a 15-page film draft, which was split into three different drafts by Han. Bong said he was inspired by the Korean film The Housemaid (1960) and the 1930s Christine and Léa Papin incident. Filming ran from May to September 2018 and included cinematographer Hong Kyung-pyo, editor Yang Jin-mo, and composer Jung Jae-il. Parasite premiered on 21 May 2019 at the Cannes Film Festival, where it became the first Korean film to win the Palme d'Or. It was praised for its direction, screenplay, performances, editing, and production design. It was released in South Korea by CJ Entertainment on 30 May, grossing $258 million worldwide on an $11.4 million budget. Among its numerous accolades, Parasite won the Academy Award for Best Picture at the 92nd Academy Awards, becoming the first non-English-language film to do so. It won an additional three Oscars, for Best Director, Best Original Screenplay, and Best International Feature Film. It is the first South Korean film to receive any Academy Award recognition, and one of only four films overall to win both the Palme d'Or and the Academy Award for Best Picture, the first such achievement in over 60 years. It won the Golden Globe Award for Best Foreign Language Film and the BAFTA Award for Best Film Not in the English Language, and became the first non-English-language film to win the Screen Actors Guild Award for Outstanding Performance by a Cast in a Motion Picture. Since its release, Parasite has been regarded as Bong's magnum opus and was ranked in the 2022 Sight and Sound poll as the 90th best film of all time. In 2025, The New York Times ranked it as the best movie of the 21st century so far in a list based on a vote by readers, actors, and directors. As of February 2025, an HBO limited television series based on the film is in early development.

The Godfather is a 1972 American epic gangster film directed by Francis Ford Coppola, who co-wrote the screenplay with Mario Puzo based on Puzo's best-selling 1969 novel. The film features an ensemble cast that includes Marlon Brando, Al Pacino, James Caan, Richard Castellano, Robert Duvall, Sterling Hayden, John Marley, Richard Conte and Diane Keaton. It is the first installment in The Godfather trilogy, which chronicles the Corleone family under patriarch Vito Corleone (Brando) and the transformation of his youngest son, Michael Corleone (Pacino), from reluctant family outsider to ruthless mafia boss. Paramount Pictures obtained the rights to the novel for $80,000, before it gained popularity. Studio executives had trouble finding a director; the first few candidates turned down the position before Coppola signed on to direct the film, but disagreement followed over casting several characters, in particular Vito (Brando) and Michael (Pacino). Filming took place primarily in locations around New York City and Sicily, and it was completed ahead of schedule. The score was composed principally by Nino Rota, with additional pieces by Carmine Coppola. The Godfather premiered at the Loew's State Theatre on March 14, 1972, and was widely released in the United States on March 24, 1972. It was the highest-grossing film of 1972, and was for a time the highest-grossing film ever made, earning between $250 and $291 million at the box office. The film was acclaimed by critics and audiences, who praised its performances—particularly those of Brando and Pacino—direction, screenplay, story, cinematography, editing, score, and portrayal of the mafia. The Godfather launched the successful careers of Coppola, Pacino, and other relative newcomers in the cast and crew. At the 45th Academy Awards, the film won Best Picture, Best Actor (Brando), and Best Adapted Screenplay (for Puzo and Coppola). In addition, the seven other Oscar nominations included Pacino, Caan, and Duvall, all for Best Supporting Actor, and Coppola for Best Director. The Godfather is regarded as one of the greatest and most influential films ever made, as well as a landmark of the gangster genre. It was selected for preservation in the U.S. National Film Registry of the Library of Congress in 1990, being deemed "culturally, historically, or aesthetically significant" and is ranked the second-greatest film in American cinema (behind Citizen Kane) by the American Film Institute. It was followed by sequels The Godfather Part II (1974) and The Godfather Part III (1990).

The Shawshank Redemption is a 1994 American drama film written and directed by Frank Darabont, based on the 1982 Stephen King novella Rita Hayworth and Shawshank Redemption. The film tells the story of banker Andy Dufresne (Tim Robbins), who is sentenced to life in Shawshank State Penitentiary for the murders of his wife and her lover, despite his claims of innocence. Over the following two decades, he befriends a fellow prisoner, contraband smuggler Ellis "Red" Redding (Morgan Freeman), and becomes instrumental in a money laundering operation led by the prison warden Samuel Norton (Bob Gunton). William Sadler, Clancy Brown, Gil Bellows, and James Whitmore appear in supporting roles. Darabont purchased the film rights to King's story in 1987, but development did not begin until five years later, when he wrote the script over eight weeks. Two weeks after submitting his script to Castle Rock Entertainment, Darabont secured a $25 million budget to produce The Shawshank Redemption, which started pre-production in January 1993. While the film is set in Maine, principal photography took place from June to August 1993 almost entirely in Mansfield, Ohio, with the Ohio State Reformatory serving as the eponymous penitentiary. The project attracted many stars for the role of Andy, including Tom Hanks, Tom Cruise, and Kevin Costner. Thomas Newman provided the film's score. The Shawshank Redemption premiered at the Toronto International Film Festival on September 10, 1994, and was released in the United States by Columbia Pictures on September 23. While the film received critical acclaim upon its release—particularly for its story and the performances of Robbins and Freeman—it was a box-office disappointment, earning only $16 million during its initial theatrical run. Its failure was attributed at the time to many factors, including competition from the films Pulp Fiction and Forrest Gump, the general unpopularity of prison films, its lack of female characters, and even the title, which was considered confusing for audiences. It went on to receive multiple award nominations, including seven nominations at the 67th Academy Awards, and a theatrical re-release that, combined with international takings, increased the film's box-office gross to $73.3 million. Over 320,000 VHS rental copies were shipped throughout the United States, and on the strength of its award nominations and word of mouth, it became the top video rental of 1995. The broadcast rights were acquired following the purchase of Castle Rock Entertainment by Turner Broadcasting System, and it was shown regularly on the TNT network starting in 1997, further increasing its popularity. Decades after release, the film is still broadcast regularly and is popular in several countries, with audience members and celebrities citing it as a source of inspiration or naming it a favorite in various surveys, leading to its recognition as one of the most beloved films among audiences. In 2015, the United States Library of Congress selected the film for preservation in the National Film Registry, finding it "culturally, historically, or aesthetically significant".

Alien is a 1979 science fiction horror film directed by Ridley Scott and written by Dan O'Bannon, based on a story by O'Bannon and Ronald Shusett. It follows a commercial starship crew who investigate a derelict space vessel and are hunted by a deadly extraterrestrial creature. The film stars Tom Skerritt, Sigourney Weaver, Veronica Cartwright, Harry Dean Stanton, John Hurt, Ian Holm, and Yaphet Kotto. It was produced by Gordon Carroll, David Giler, and Walter Hill through their company Brandywine Productions and was distributed by 20th Century-Fox. Giler and Hill revised and made additions to the script; Shusett was the executive producer. The alien creatures and environments were designed by the Swiss artist H. R. Giger, while the concept artists Ron Cobb and Chris Foss designed the other sets. Alien premiered on May 24, 1979 at the fourth Seattle International Film Festival. It received a wide release on June 22 and was released on September 6 in the United Kingdom. It initially received mixed reviews, but won the Academy Award for Best Visual Effects, three Saturn Awards (Best Science Fiction Film, Best Direction for Scott, and Best Supporting Actress for Cartwright), and a Hugo Award for Best Dramatic Presentation. Alien grossed $78.9 million in the United States and £7.8 million in the United Kingdom during its first theatrical run. Its worldwide gross has been estimated at between $109 million and $188 million. In subsequent years, Alien was critically reassessed and described as one of the greatest and most influential science fiction and horror films of all time. In 2002, Alien was deemed "culturally, historically, or aesthetically significant" by the Library of Congress and was selected for preservation in the United States National Film Registry. In 2008, it was ranked by the American Film Institute as the seventh-best film in the science fiction genre, and as the 33rd-greatest film of all time by Empire. The success of Alien spawned a media franchise of films, books, video games, and toys, and propelled Weaver's acting career. The story of her character's encounters with the alien creatures became the thematic and narrative core of the sequels Aliens (1986), Alien 3 (1992), and Alien Resurrection (1997). A crossover with the Predator franchise produced the Alien vs. Predator films, while a two-film prequel series was directed by Scott before Alien: Romulus (2024), a standalone sequel, was released. A television prequel written by Noah Hawley and produced by Scott, Alien: Earth, was released on FX on Hulu on August 12, 2025.

Apocalypse Now is a 1979 American psychological epic war film produced and directed by Francis Ford Coppola (credited as Francis Coppola). The screenplay, written by Coppola and John Milius, with narration by Michael Herr, is loosely inspired by the 1899 novella Heart of Darkness by Joseph Conrad, with the setting changed from late 19th-century Congo to the Vietnam War. The film follows a river journey from South Vietnam into Cambodia undertaken by Captain Willard (Martin Sheen), who is on a secret mission to assassinate Colonel Kurtz (Marlon Brando), a renegade Special Forces officer who is accused of murder and presumed insane. The ensemble cast also features Robert Duvall, Frederic Forrest, Albert Hall, Sam Bottoms, Laurence Fishburne, Dennis Hopper, G. D. Spradlin and Harrison Ford. Milius became interested in adapting Heart of Darkness for a Vietnam War setting in the late 1960s, and initially began developing the film with Coppola as producer and George Lucas as director. After Lucas became unavailable, Coppola took over directorial control, and was influenced by Werner Herzog's Aguirre, the Wrath of God (1972) in his approach to the material. Initially set to be a five-month shoot in the Philippines starting in March 1976, a series of problems lengthened it to over a year. These problems included expensive sets being destroyed by severe weather, Brando arriving on set overweight and completely unprepared, and Sheen having a breakdown and suffering a near-fatal heart attack on location. After photography was finally finished in May 1977, the release was postponed several times while Coppola edited over a million feet of film. Many of these difficulties are chronicled in the documentary Hearts of Darkness: A Filmmaker's Apocalypse (1991). Apocalypse Now was honored with the Palme d'Or at the Cannes Film Festival, where it premiered unfinished. When it was finally released on August 15, 1979, by United Artists, it performed well at the box office, grossing $80 million in the United States and Canada and $150 million worldwide. Initial reviews were mixed; while Vittorio Storaro's cinematography was widely acclaimed, several critics found Coppola's handling of the story's major themes anticlimactic and intellectually disappointing. The film was nominated for eight Academy Awards, including Best Picture, Best Director (Coppola), and Best Supporting Actor (Duvall); it went on to win Best Cinematography and Best Sound. Apocalypse Now has been assessed as Coppola's magnum opus and retrospectively considered one of the greatest films ever made. In 2000, the film was selected for preservation in the National Film Registry by the U.S. Library of Congress as "culturally, historically or aesthetically significant". Coppola later released Apocalypse Now Redux, an extended re-edit of the film that contains multiple new scenes, in 2001. Another re-edit, Apocalypse Now Final Cut, was released in 2019 and is Coppola's preferred version of the film.

Star Wars is an American media franchise created by George Lucas. The space opera saga began with the original Star Wars film (1977) and quickly became a worldwide pop culture phenomenon. It has expanded into various films and other media, including television series, video games, novels, comic books, theme park attractions, and themed areas, comprising an all-encompassing fictional universe. Star Wars is one of the highest-grossing media franchises of all time. The original film, later retitled Episode IV: A New Hope, was followed by the sequels Episode V: The Empire Strikes Back (1980) and Episode VI: Return of the Jedi (1983), forming the original Star Wars trilogy. Lucas later returned to the series to write and direct a prequel trilogy, consisting of Episode I: The Phantom Menace (1999), Episode II: Attack of the Clones (2002), and Episode III: Revenge of the Sith (2005). In 2012, Lucas sold his production company to Disney, relinquishing his ownership of the franchise. This led to a sequel trilogy, consisting of Episode VII: The Force Awakens (2015), Episode VIII: The Last Jedi (2017), and Episode IX: The Rise of Skywalker (2019). All nine films, collectively referred to as the "Skywalker Saga", were nominated for Academy Awards, with Oscars going to the first three releases. Together with the spin-off films Rogue One (2016), Solo (2018) and The Mandalorian and Grogu (2026), the combined box office revenue of Star Wars theatrical live-action films equals over US$10 billion, making Star Wars the third-highest-grossing film franchise in cinematic history.

Pulp Fiction is a 1994 American crime film written and directed by Quentin Tarantino from a story he conceived with Roger Avary. It tells four intertwining tales of crime and violence in Los Angeles. The film stars John Travolta, Samuel L. Jackson, Bruce Willis, Tim Roth, Ving Rhames, and Uma Thurman. The title refers to the pulp magazines and hardboiled crime novels popular during the mid-20th century, known for their graphic violence and punchy dialogue. Tarantino wrote Pulp Fiction in 1992 and 1993, incorporating scenes that Avary originally wrote for True Romance (1993). Its plot occurs out of chronological order. The film features self-referential elements, monologues, casual conversations with eclectic dialogue, and an ironic combination of humor and violence. TriStar Pictures turned down the script as "too demented". Miramax Films co-chairman Harvey Weinstein was enthralled, however, and the film became the first that Miramax Films fully financed. Pulp Fiction won the Palme d'Or at the 1994 Cannes Film Festival and was a major critical and commercial success. It was nominated for seven awards at the 67th Academy Awards, including Best Picture, and won Best Original Screenplay; Travolta, Jackson, and Thurman were nominated for Best Actor, Best Supporting Actor, and Best Supporting Actress respectively. As a result of the film's success, Travolta's career was reinvigorated. The film's development, marketing, distribution, and profitability had a sweeping effect on independent cinema. Pulp Fiction is regarded as Tarantino's magnum opus, with particular praise for its screenwriting. The self-reflexivity, unconventional structure, and extensive homage and pastiche have led critics to describe it as a touchstone of postmodern film. It is considered a cultural watershed, influencing films and other media that adopted elements of its style. The cast was also widely praised, with Travolta, Thurman, and Jackson earning high acclaim. In 2008, Entertainment Weekly named it the best film since 1983 and it has appeared on many critics' lists of the greatest films. In 2013, Pulp Fiction was selected for preservation in the US National Film Registry by the Library of Congress as "culturally, historically, or aesthetically significant".

Casablanca is a 1942 American romantic drama film directed by Michael Curtiz and starring Humphrey Bogart, Ingrid Bergman, and Paul Henreid. Filmed and set during World War II, it focuses on an American expatriate (Bogart) who must choose between his love for a woman (Bergman) and helping her husband (Henreid), a Czechoslovak resistance leader, escape from the Vichy-controlled city of Casablanca to continue his fight against the Nazis. The screenplay is based on Everybody Comes to Rick's, an unproduced stage play by Murray Burnett and Joan Alison. The supporting cast features Claude Rains, Conrad Veidt, Sydney Greenstreet, Peter Lorre, and Dooley Wilson. Warner Bros. story editor Irene Diamond convinced producer Hal B. Wallis to purchase the film rights to the play in January 1942. Brothers Julius and Philip G. Epstein were initially assigned to write the script. However, despite studio resistance, they left to work on Frank Capra's Why We Fight series early in 1942. Howard Koch was assigned to the screenplay until the Epsteins returned a month later. Principal photography began on May 25, 1942, ending on August 3; the film was shot entirely at Warner Bros. Studios in Burbank, California, with the exception of one sequence at Van Nuys Airport in Los Angeles. Although Casablanca was an A-list film with established stars and first-rate writers, no one involved with its production expected it to stand out among the many pictures produced by Hollywood yearly. Casablanca was rushed into release to take advantage of the publicity from the Allied invasion of North Africa a few weeks earlier. It had its world premiere on November 26, 1942, in New York City and was released nationally in the United States on January 23, 1943. The film was a solid, if unspectacular, success in its initial run. Exceeding expectations, Casablanca went on to win the Academy Award for Best Picture, while Curtiz was selected as Best Director and the Epsteins and Koch were honored for Best Adapted Screenplay. Its reputation has gradually grown, to the point that its lead characters, memorable lines, and pervasive theme song have all become iconic, and it consistently ranks near the top of lists of the greatest films in history. In the inaugural class of 1989, the United States Library of Congress selected the film as one of the first for preservation in the National Film Registry for being "culturally, historically, or aesthetically significant". The film's copyright was renewed in 1970; it will enter the public domain in the US and countries with the rule of the shorter term on January 1, 2038. Roger Ebert wrote: "If there is ever a time when they decide that some movies should be spelled with an upper-case M, Casablanca should be voted first on the list of Movies."

The Dark Knight is a 2008 superhero film directed by Christopher Nolan, from a screenplay co-written with his brother Jonathan. Based on the DC Comics superhero Batman, it is the sequel to Batman Begins (2005), and the second installment in The Dark Knight trilogy. The plot follows the vigilante Batman, police lieutenant James Gordon, and district attorney Harvey Dent, who form an alliance to dismantle organized crime in Gotham City. Their efforts are derailed by the Joker, an anarchistic mastermind who seeks to test how far Batman will go to save the city from chaos. The ensemble cast includes Christian Bale, Michael Caine, Heath Ledger, Gary Oldman, Aaron Eckhart, Maggie Gyllenhaal, and Morgan Freeman. Warner Bros. Pictures prioritized a sequel following the successful reinvention of the Batman film series with Batman Begins. Christopher Nolan and Batman Begins co-writer David S. Goyer developed the story elements, making Dent the central protagonist caught up in the battle between Batman and the Joker. In writing the screenplay, the Nolans were influenced by 1980s Batman comics and crime drama films, and sought to continue Batman Begins' heightened sense of realism. From April to November 2007, filming took place with a $185 million budget in Chicago and Hong Kong, and on sets in England. The Dark Knight was the first major motion picture to be filmed with high-resolution IMAX cameras. Nolan avoided using computer-generated imagery unless necessary, insisting on practical stunts such as flipping an 18-wheel truck and blowing up a factory. The Dark Knight was marketed with an innovative interactive viral campaign that initially focused on countering criticism of Ledger's casting by those who believed he was a poor choice to portray the Joker. Ledger died from an accidental prescription drug overdose in January 2008, leading to widespread interest from the press and public regarding his performance. When it was released in July, The Dark Knight received acclaim for its mature tone and themes, visual style, and performances—particularly that of Ledger, who received many posthumous awards including Academy, BAFTA, and Golden Globe awards for Best Supporting Actor. This made The Dark Knight the first comic-book film to receive major acting awards. It broke several box-office records and became the highest-grossing 2008 film, the fourth-highest-grossing film to that time, and the highest-grossing superhero film of the time. Since its release, The Dark Knight has been assessed as one of the greatest superhero films ever, one of the best movies of the 2000s, and one of the best films ever made. It is considered the "blueprint" for many modern superhero films, particularly for its rejection of a typical comic-book movie style in favor of a crime film that features comic-book characters. Many filmmakers attempted to replicate its success by adopting a similarly gritty, realistic tone, though with uneven results. The Dark Knight has been analyzed for its themes of terrorism and the limitations of morality and ethics. The United States Library of Congress selected it for preservation in the National Film Registry in 2020. A sequel, The Dark Knight Rises, concluded The Dark Knight trilogy in 2012.

Blade Runner is a 1982 science fiction film directed by Ridley Scott from a screenplay by Hampton Fancher and David Peoples. Starring Harrison Ford, Rutger Hauer, Sean Young, and Edward James Olmos, it is an adaptation of Philip K. Dick's 1968 novel Do Androids Dream of Electric Sheep? The film is set in a dystopian future Los Angeles of 2019, in which synthetic humans known as replicants are bio-engineered by the powerful Tyrell Corporation to work on space colonies. When a fugitive group of advanced replicants led by Roy Batty (Hauer) escapes back to Earth, former cop Rick Deckard (Ford) is recalled to hunt them down. Blade Runner initially underperformed in North American theaters and polarized critics; some praised its thematic complexity and visuals, while others critiqued its slow pacing and lack of action. The film's soundtrack, composed by Vangelis, was nominated in 1982 for a BAFTA and a Golden Globe as best original score. Blade Runner later became a cult film, and has since come to be regarded as one of the greatest science fiction films. Hailed for its production design depicting a high-tech but decaying future, the film is often regarded as both a leading example of neo-noir cinema and a foundational work of the cyberpunk genre. It has influenced many science fiction films, video games, anime, and television series. It also brought the work of Dick to Hollywood's attention and led to several film adaptations of his works. In 1993, it was selected for preservation in the National Film Registry by the Library of Congress. Seven different versions of Blade Runner exist as a result of controversial changes requested by studio executives. A director's cut was released in 1992 after a strong response to test screenings of a workprint. This, in conjunction with the film's popularity as a video rental, made it one of the earliest films to be released on DVD. In 2007, Warner Bros. released The Final Cut, a 25th-anniversary digitally remastered version; this is the only version over which Scott retained artistic control. The film is the first of the franchise of the same name. A sequel, titled Blade Runner 2049, was released in 2017 alongside a trilogy of short films covering the thirty-year span between the two films' settings. The anime series Blade Runner: Black Lotus was released in 2021.

The Lord of the Rings: The Fellowship of the Ring is a 2001 epic fantasy film directed by Peter Jackson from a screenplay by Fran Walsh, Philippa Boyens, and Jackson. It is based on J. R. R. Tolkien's 1954 The Fellowship of the Ring, the first volume of the novel The Lord of the Rings. The film is the first instalment in The Lord of the Rings trilogy. It features an ensemble cast including Elijah Wood, Ian McKellen, Liv Tyler, Viggo Mortensen, Sean Astin, Cate Blanchett, John Rhys-Davies, Billy Boyd, Dominic Monaghan, Orlando Bloom, Christopher Lee, Hugo Weaving, Sean Bean, Ian Holm, and Andy Serkis. Set in Middle-earth, the story tells of the Dark Lord Sauron, who seeks the One Ring, which contains part of his might, to return to power. The Ring has found its way to the young hobbit Frodo Baggins. The fate of Middle-earth hangs in the balance as Frodo and eight companions (who form the eponymous Fellowship of the Ring) begin their perilous journey to Mount Doom in the land of Mordor, the only place where the Ring can be destroyed. The Fellowship of the Ring was financed and distributed by American studio New Line Cinema, but filmed and edited entirely in Jackson's native New Zealand, concurrently with the other two parts of the trilogy. It premiered on 10 December 2001 at the Odeon Leicester Square in London and was released on 19 December in the United States and on 20 December in New Zealand. The film was acclaimed by critics and audiences, who considered it a landmark in filmmaking and an achievement in the fantasy film genre, with praise for the visual effects, performances, direction, screenplay, musical score, and faithfulness to the source material. It grossed $868 million worldwide during its original theatrical run, making it the second-highest-grossing film of 2001 and the fifth-highest-grossing film of all time at the time of its release. Following subsequent reissues, it has grossed $897 million. Like its successors, The Fellowship of the Ring is widely recognised as one of the greatest and most influential films ever made. The film received numerous accolades; at the 74th Academy Awards, it was nominated for thirteen awards, including Best Picture, winning for Best Cinematography, Best Makeup, Best Original Score, and Best Visual Effects. In 2007, the American Film Institute named it one of the 100 greatest American films in history, being both the most recent film and the only film released in the 21st century to make it to the list. In 2021, the film was selected for preservation in the United States National Film Registry by the Library of Congress for being "culturally, historically, or aesthetically significant". Two sequels, The Two Towers and The Return of the King, followed in 2002 and 2003, respectively.

Nelson Rolihlahla Mandela ( man-DEL-ə, Xhosa: [xolíɬaɬa mandɛ̂ːla]; born Rolihlahla Mandela; 18 July 1918 – 5 December 2013) was a South African anti-apartheid activist, statesman, and revolutionary who was the first president of South Africa from 1994 to 1999. He was the country's first Black head of state and the first elected in a fully representative democratic election. His administration focused on dismantling the legacy of apartheid by fostering racial reconciliation, a national peace accord and eventual multiracial democracy. Ideologically an African nationalist and socialist, he served as the president of the African National Congress (ANC) party from 1991 to 1997. Mandela was born into the Thembu royal family in Mvezo, South Africa. He studied law at the University of Fort Hare and the University of Witwatersrand before working as a lawyer in Johannesburg. There he became involved in anti-colonial and African nationalist politics, joining the ANC in 1943 and co-founding its Youth League in 1944. After the National Party's white-only government established apartheid, a system of racial segregation that privileged whites, Mandela and the ANC committed themselves to its overthrow. Rising to prominence for his involvement in the 1952 Defiance Campaign and the 1955 Congress of the People, he was repeatedly arrested for seditious activities and was unsuccessfully prosecuted in the 1956 Treason Trial. Influenced by Marxism, he secretly joined the banned South African Communist Party (SACP). Although initially committed to non-violent protest, in association with the SACP he co-founded the militant uMkhonto we Sizwe in 1961 that led a sabotage campaign against the apartheid government. Following the Rivonia Trial, he was sentenced to life imprisonment for conspiring to overthrow the state. Mandela served 27 years in prison. Amid growing domestic and international pressure and fears of racial civil war, President F. W. de Klerk released him in 1990. Mandela and de Klerk led efforts to negotiate an end to apartheid, which resulted in the 1994 multiracial general election in which Mandela became president. Leading a broad coalition government which promulgated a new constitution, Mandela created the Truth and Reconciliation Commission to investigate past human rights abuses. Economically, his administration retained its predecessor's liberal framework despite his own socialist beliefs, also introducing measures to encourage land reform, combat poverty and expand healthcare services. Internationally, Mandela acted as mediator in the Pan Am Flight 103 bombing trial and served as secretary-general of the Non-Aligned Movement from 1998 to 1999. He declined a second presidential term and was succeeded by Thabo Mbeki. Mandela focused on combating poverty and HIV/AIDS through the charitable Nelson Mandela Foundation. Mandela was a controversial figure for much of his life. Although critics on the right denounced him as a communist terrorist and critics on the far left deemed him too eager to negotiate and reconcile with apartheid's supporters, he gained international acclaim for his activism. Globally regarded as an icon of moral leadership, peace, democracy and social justice, he received more than 250 honours, including the Nobel Peace Prize. He is held in deep respect within South Africa, where he is often referred to by his Thembu clan name, Madiba, and described as the "Father of the Nation". Mandela is widely considered one of the greatest and most admired figures of the 20th century.

Michelangelo di Lodovico Buonarroti Simoni (6 March 1475 – 18 February 1564), known mononymously as Michelangelo, was an Italian sculptor, painter, architect, and poet of the High Renaissance. He was born in the Republic of Florence but was mostly active in Rome from his 30s onwards. His work was inspired by models from classical antiquity and had a lasting influence on Western art. Michelangelo's creative abilities and mastery in a range of artistic arenas define him as an archetypal Renaissance man, along with his rival and elder contemporary, Leonardo da Vinci. Given the sheer volume of surviving correspondence, sketches, and reminiscences, Michelangelo is one of the best-documented artists of the 16th century. He was lauded by contemporary biographers as the most accomplished artist of his era. Michelangelo achieved fame early. Two of his best-known works, the Pietà and David, were sculpted before the age of 30. Although he did not consider himself a painter, Michelangelo created two of the most influential frescoes in the history of Western art: the scenes from Genesis on the ceiling of the Sistine Chapel in Rome, and The Last Judgment on its altar wall. His design of the Laurentian Library pioneered Mannerist architecture. At the age of 71, he succeeded Antonio da Sangallo the Younger as the architect of St. Peter's Basilica. Michelangelo transformed the plan so that the Western end was finished to his design, as was the dome, with some modification, after his death. Michelangelo was the first Western artist whose biography was published while he was alive. Three biographies were published during his lifetime. One of them, by Giorgio Vasari, proposed that Michelangelo's work transcended that of any artist living or dead, and was "supreme in not one art alone but in all three". In his lifetime, Michelangelo was often called Il Divino ("the divine one"). His contemporaries admired his terribilità—his ability to instill a sense of awe in viewers of his art. Attempts by subsequent artists to imitate the expressive physicality of Michelangelo's style contributed to the rise of Mannerism, a short-lived movement in Western art between the High Renaissance and the Baroque.

Charles John Huffam Dickens ( ; 7 February 1812 – 9 June 1870) was an English writer and journalist. He created some of literature's best-known fictional characters, and is regarded by many as the greatest novelist of the Victorian era. His works enjoyed unprecedented popularity during his lifetime and, by the 20th century, critics and scholars had recognised him as a literary genius. His novels and short stories are widely read today. Born in Portsmouth, Dickens left school at age 12 to work in a boot-blacking factory when his father John was incarcerated in a debtors' prison. After three years, he returned to school before beginning his literary career as a journalist. Dickens edited a weekly journal for 20 years; wrote 15 novels, 5 novellas, hundreds of short stories and nonfiction articles; lectured and performed readings extensively; was a tireless letter writer; and campaigned vigorously for children's rights, education and other social reforms. Dickens's literary success began with the 1836 serial publication of The Pickwick Papers, a publishing phenomenon—thanks largely to the introduction of the character Sam Weller in the fourth episode—that sparked Pickwick merchandise and spin-offs. Within a few years, Dickens had become an international literary celebrity, famous for his humour, satire and keen observation of character and society. His novels, most of them published in monthly or weekly instalments, pioneered the serial publication of narrative fiction, which became the dominant Victorian mode for novel publication. Cliffhanger endings in his serial publications kept readers in suspense. The installment format allowed Dickens to evaluate his audience's reaction, and he often modified his plot and character development based on such feedback. For example, when his wife's podiatrist expressed distress at the way Miss Mowcher in David Copperfield seemed to reflect her own disabilities, Dickens improved the character with positive features. His plots were carefully constructed and he often wove elements from topical events into his narratives. Masses of the illiterate poor would individually pay a halfpenny to have each new monthly episode read to them, opening up and inspiring a new class of readers. His 1843 novella A Christmas Carol remains especially popular and continues to inspire adaptations in every creative medium. Oliver Twist and Great Expectations are also frequently adapted and, like many of his novels, evoke images of early Victorian London. His 1853 novel Bleak House, a satire on the judicial system, helped support a reformist movement that culminated in the 1870s legal reform in England. A Tale of Two Cities (1859; set in London and Paris) is regarded as his best-known work of historical fiction. The most famous celebrity of his era, he undertook, in response to public demand, a series of public reading tours in the later part of his career. The term Dickensian is used to describe something that is reminiscent of Dickens and his writings, such as poor social or working conditions, or comically repulsive characters.

Augusta Ada King, Countess of Lovelace (née Byron; 10 December 1815 – 27 November 1852), also known as Ada Lovelace, was an English mathematician and writer chiefly known for work on Charles Babbage's proposed mechanical general-purpose computer, the analytical engine. She was the first to recognise the machine had applications beyond pure calculation. Lovelace is often considered the first computer programmer. Lovelace was the only legitimate child of poet Lord Byron and reformer Anne Isabella Milbanke. Lord Byron separated from his wife a month after Ada was born, and died when she was eight. Although often ill in childhood, Lovelace pursued her studies assiduously. She married William King in 1835. King was a Baron, and was created Viscount Ockham and 1st Earl of Lovelace in 1838. The name Lovelace was chosen because Ada was descended from the extinct Baron Lovelaces. The title given to her husband thus made Ada the Countess of Lovelace. Lovelace's educational and social exploits brought her into contact with scientists such as Andrew Crosse, Charles Babbage, David Brewster, Charles Wheatstone and Michael Faraday, and the author Charles Dickens, contacts which she used to further her education. Lovelace described her approach as "poetical science" and herself as an "Analyst (& Metaphysician)". When she was eighteen, Lovelace's mathematical talents led her to a long working relationship and friendship with fellow British mathematician Charles Babbage. She was particularly interested in Babbage's work on the analytical engine. Lovelace first met him on 5 June 1833, when she and her mother attended one of Charles Babbage's Saturday night soirées with their mutual friend, and Lovelace's private tutor, Mary Somerville. Though Babbage's analytical engine was never constructed and did not influence the invention of electronic computers, it has been recognised as a Turing-complete general-purpose computer, which anticipated the essential features of a modern electronic computer. Babbage is therefore known as the "father of computers," and Lovelace is credited with several computing "firsts" for her collaboration with him. Lovelace translated an article by the military engineer Luigi Menabrea about the analytical engine, supplementing it with seven long explanatory notes. These described a method of using the machine to calculate Bernoulli numbers which is often called the first published computer program. She developed a vision of the capability of computers to go beyond mere calculating or number-crunching, while many others, including Babbage, focused only on those capabilities. Lovelace was the first to point out the possibility of encoding information besides mere arithmetical figures, such as music, and manipulating it with such a machine. Her mindset of "poetical science" led her to ask questions about the analytical engine, examining how individuals and society relate to technology as a collaborative tool. Ada is widely commemorated, including in the names of a programming language, roads, buildings and institutes, as well as programmes, lectures and courses. There are plaques, statues, paintings, literary and non-fiction works about her.

Pablo Ruiz Picasso (25 October 1881 – 8 April 1973) was a Spanish painter and sculptor who spent most of his adult life in France. One of the most influential artists of the 20th century, he is known for co-founding the Cubist movement, the invention of constructed sculpture, the co-invention of collage, and for the wide variety of styles that he helped develop and explore. Among his most famous works are the proto-Cubist Les Demoiselles d'Avignon (1907) and the anti-war painting Guernica (1937), a dramatic portrayal of the bombing of Guernica by German and Italian air forces during the Spanish Civil War. His career spanned more than 76 years, from his late teens to his death in 1973. Beginning his formal training under his father José Ruiz y Blasco aged seven, Picasso demonstrated extraordinary artistic talent from a young age, painting in a naturalistic manner through his childhood and adolescence. During the first decade of the 20th century, his style changed as he experimented with different theories, techniques, and ideas. After 1906, the Fauvist work of the older artist Henri Matisse motivated Picasso to explore more radical styles, beginning a fruitful rivalry between the two artists, who subsequently were often paired by critics as the leaders of modern art. Picasso's output, especially in his early career, is often periodized. While the names of many of his later periods are debated, the most commonly accepted periods in his work are the Blue Period (1901–1904), the Rose Period (1904–1906), the African-influenced Period (1907–1909), Analytic Cubism (1909–1912), and Synthetic Cubism (1912–1919), also referred to as the Crystal period. Much of Picasso's work of the late 1910s and early 1920s is in a neoclassical style, and his work in the mid-1920s often has characteristics of Surrealism. His later work often combines elements of his earlier styles. Exceptionally prolific throughout the course of his long life, Picasso achieved universal renown and immense fortune for his revolutionary artistic accomplishments, and became one of the best-known figures in 20th-century art.

Albert Einstein (14 March 1879 – 18 April 1955) was a German-born theoretical physicist best known for developing the known theory of relativity. Einstein also made important contributions to quantum theory. His mass–energy equivalence formula E = mc2, which arises from special relativity, has been called "the world's most famous equation". He received the 1921 Nobel Prize in Physics for "his services to theoretical physics, and especially for his discovery of the law of the photoelectric effect". Born as a subject to the Kingdom of Württemberg, part of the German Empire, Einstein moved to Switzerland in 1895, forsaking his citizenship the following year. In 1897, at the age of seventeen, he enrolled in the mathematics and physics teaching diploma program at the Swiss federal polytechnic school in Zurich, graduating in 1900. He acquired Swiss citizenship a year later, which he kept for the rest of his life, and afterwards secured a permanent position at the Swiss Patent Office in Bern. In 1905, he submitted a successful PhD dissertation to the University of Zurich. In 1914, he moved to Berlin to join the Prussian Academy of Sciences and the Humboldt University of Berlin, becoming director of the Kaiser Wilhelm Institute for Physics in 1917; he also became a Prussian and consequently also German citizen again. In 1933, while Einstein was visiting the United States, Adolf Hitler came to power in Germany. Horrified by the Nazi persecution of his fellow Jews, he decided to remain in the US, and was granted American citizenship in 1940. On the eve of World War II, he endorsed a letter to President Franklin D. Roosevelt alerting him to the potential German nuclear weapons program and recommending that the US begin similar research, later carried out as the Manhattan Project. In 1905, sometimes described as his annus mirabilis (miracle year), he published four groundbreaking papers. In them, he outlined a theory of the photoelectric effect, explained Brownian motion, introduced his special theory of relativity, and demonstrated that if the special theory is correct, mass and energy are equivalent to each other. In 1915, he proposed a general theory of relativity that extended his system of mechanics to incorporate gravitation. A paper that he published the following year laid out the implications of general relativity for the modeling of the structure and evolution of the universe as a whole. It introduced the cosmological constant and is further regarded as the first step in the field of modern theoretical cosmology. In 1917, Einstein wrote a paper which introduced the concepts of spontaneous emission and stimulated emission, the latter of which is the core mechanism behind the laser and maser, and which contained a trove of information that would be beneficial to developments in physics later on, such as quantum electrodynamics and quantum optics. In the middle part of his career, Einstein made important contributions to statistical mechanics and quantum theory. Especially notable was his work on the quantum physics of radiation, in which light consists of particles, subsequently called photons. With physicist Satyendra Nath Bose, he laid the groundwork for Bose–Einstein statistics. For much of the last phase of his academic life, Einstein worked on two endeavors that ultimately proved unsuccessful. First, he advocated against quantum theory's introduction of fundamental randomness into science's picture of the world, objecting that "God does not play dice". Second, he attempted to devise a unified field theory by generalizing his geometric theory of gravitation to include electromagnetism. As a result, he became increasingly isolated from mainstream modern physics. Many things are named after him, including the element Einsteinium. In 1999, he was named Time's Person of the Century.

Charles Robert Darwin ( DAR-win; 12 February 1809 – 19 April 1882) was an English naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all species of life have descended from a common ancestor is now generally accepted and considered a fundamental scientific concept. In a joint presentation with Alfred Russel Wallace, he introduced his scientific theory that this branching pattern of evolution resulted from a process he called natural selection, in which the struggle for existence has a similar effect to the artificial selection involved in selective breeding. Darwin has been described as one of the most influential figures in human history and was honoured by burial in Westminster Abbey. Darwin's early interest in nature led him to neglect his medical education at the University of Edinburgh; instead, he helped Robert Edmond Grant to investigate marine invertebrates. His studies at the University of Cambridge's Christ's College from 1828 to 1831 encouraged his passion for natural science. However, it was his five-year voyage on HMS Beagle from 1831 to 1836 that truly established Darwin as an eminent geologist. The observations and theories he developed during his voyage supported Charles Lyell's concept of gradual geological change. Publication of his journal of the voyage made Darwin famous as a popular author. His first scientific work was The Structure and Distribution of Coral Reefs (1842). Along with his work on barnacles, it won him the Royal Medal in 1853. Puzzled by the geographical distribution of wildlife and fossils he collected on the voyage, Darwin began detailed investigations and, in 1838, devised his theory of natural selection. Although he discussed his ideas with several naturalists, he needed time for extensive research, and his geological work had priority. He was writing up his theory in 1858 when Wallace sent him an essay that described the same idea, prompting the immediate joint submission of both their theories to the Linnean Society of London. Darwin's work established evolutionary descent with modification as the dominant scientific explanation of natural diversification. Darwin published his theory of evolution with compelling evidence in On the Origin of Species (1859). He explored coevolution in Fertilisation of Orchids (1862) and human evolution and sexual selection in The Descent of Man, and Selection in Relation to Sex (1871). The Expression of the Emotions in Man and Animals (1872) was an early work of psychology, and one of the first books to feature photographs. His final book was The Formation of Vegetable Mould, through the Actions of Worms (1881). By the 1870s, the scientific community and a majority of the educated public had accepted evolution as a fact. However, many initially favoured competing explanations that gave only a minor role to natural selection. It was not until the emergence of the modern evolutionary synthesis from the 1930s to the 1950s that a broad consensus developed in which natural selection was the basic mechanism of evolution. Darwin's discovery is the unifying theory of the life sciences, explaining the unity and diversity of life.

Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642), commonly referred to as Galileo Galilei, was an Italian astronomer, physicist, and engineer, sometimes described as a polymath. He was born in the city of Pisa, then part of the Duchy of Florence. Galileo has been called the father of observational astronomy, modern-era classical physics, the scientific method, and modern science. He studied speed and velocity, gravity and free fall, the principle of relativity, inertia, projectile motion, and also worked in applied science and technology, describing the properties of the pendulum and "hydrostatic balances". He was one of the earliest developers of the thermoscope and the inventor of various military compasses. With an improved telescope he built, he observed the stars of the Milky Way, the phases of Venus, the four largest satellites of Jupiter, Saturn's rings, lunar craters, and sunspots. He also built an early microscope. Galileo's championing of Copernican heliocentrism was met with opposition from within the Catholic Church and from some astronomers. The matter was investigated by the Roman Inquisition in 1615, which concluded that his opinions contradicted accepted Biblical interpretations. Galileo later defended his views in Dialogue Concerning the Two Chief World Systems (1632), which appeared to dispute and satirize Pope Urban VIII, thus alienating both the Pope and the Jesuits, who had both strongly supported Galileo until this point. He was tried by the Inquisition, found "vehemently suspect of heresy", and forced to recant. He spent the rest of his life under house arrest. During this time, he wrote Two New Sciences (1638), primarily concerning kinematics and the strength of materials.

Joan of Arc (French: Jeanne d'Arc [ʒan daʁk] ; Middle French: Jehanne Darc [ʒəˈãnə ˈdark]; c. 1412 – 30 May 1431) is a patron saint of France, honored as a defender of the French nation for her role in the siege of Orléans and her insistence on the coronation of Charles VII of France during the Hundred Years' War. Stating that she acted under divine guidance, she became a military leader who gained recognition as a savior of France. Joan was born to a propertied peasant family at Domrémy in northeast France. In 1428, she requested to be taken to Charles VII, later testifying that she was guided by visions from the archangel Michael, Saint Margaret, and Saint Catherine to help him save France from English domination. Convinced of her devotion and purity, Charles sent Joan, who was about seventeen years old, to Orléans as part of a relief army. She arrived at the city in April 1429, wielding her banner and bringing hope to the demoralized French army. Nine days after her arrival, the English abandoned the siege. Joan encouraged the French to aggressively pursue the English during the Loire Campaign, which culminated in another decisive victory at Patay, opening the way for the French army to advance on Reims unopposed, where Charles was crowned as the king of France with Joan at his side. These victories boosted French morale, paving the way for their final triumph in the Hundred Years' War several decades later. After Charles's coronation, Joan participated in the unsuccessful siege of Paris in September 1429 and the failed siege of La Charité in November. Her role in these defeats reduced the court's faith in her. In early 1430, Joan organized a company of volunteers to relieve Compiègne, which had been besieged by the Burgundians—French allies of the English. She was captured by Burgundian troops on 23 May. After trying unsuccessfully to escape, she was handed to the English in November. She was put on trial by Bishop Pierre Cauchon on accusations of heresy, which included blaspheming by wearing men's clothes, acting upon visions that were demonic, and refusing to submit her words and deeds to the judgment of the church. She was declared guilty and burned at the stake on 30 May 1431, aged about nineteen. In 1456, an inquisitorial court reinvestigated Joan's trial and overturned the verdict, declaring that it was tainted by deceit and procedural errors. Joan has been described as an obedient member of the Catholic Church, an early feminist, and a symbol of freedom and independence. She is popularly revered as a martyr. After the French Revolution, she became a national symbol of France. In 1920, Joan of Arc was canonized by Pope Benedict XV and, two years later, was declared one of the patron saints of France. She is portrayed in numerous cultural works, including literature, music, paintings, sculptures, and theater.

Leonardo di ser Piero da Vinci (15 April 1452 – 2 May 1519) was an Italian polymath of the High Renaissance who was active as a painter, draughtsman, engineer, scientist, theorist, sculptor, and architect. While his fame initially rested on his achievements as a painter, he has also become known for his notebooks, in which he made drawings and notes on a variety of subjects, including anatomy, astronomy, botany, cartography, painting, and palaeontology. Leonardo is widely regarded as a genius who epitomised the Renaissance humanist ideal, and his collective works contributed to the development of European art to an extent rivalled only by that of his younger contemporary Michelangelo. Born out of wedlock to a successful notary and a lower-class woman in, or near, Vinci, he was educated in Florence by the Italian painter and sculptor Andrea del Verrocchio. He began his career in the city, but then spent much time in the service of Ludovico Sforza in Milan. Later, he worked in Florence and Milan again, as well as briefly in Rome, all while attracting a large following of imitators and students. Upon the invitation of Francis I, he spent his last three years in France, where he died in 1519. Since his death, there has not been a time when his achievements, diverse interests, personal life, and empirical thinking have failed to incite interest and admiration, making him a frequent namesake and subject in culture. Leonardo is identified as one of the greatest painters in the history of Western art and is often credited as the founder of the High Renaissance. Despite having many lost works and fewer than 25 attributed major works – including numerous unfinished works – he created some of the most influential paintings in the Western canon. The Mona Lisa is his best known work and is regarded as the world's most famous individual painting. The Last Supper is the most reproduced religious painting of all time and his Vitruvian Man drawing is also regarded as a cultural icon. In 2017, Salvator Mundi, attributed in whole or part to Leonardo, was sold at auction for US$450.3 million, setting a new record for the most expensive painting ever sold at public auction. Revered for his technological ingenuity, he conceptualised flying machines, a type of armoured fighting vehicle, concentrated solar power, a ratio machine that could be used in an adding machine, and the double hull. Relatively few of his designs were constructed or were even feasible during his lifetime, as the modern scientific approaches to metallurgy and engineering were only in their infancy during the Renaissance. Some of his smaller inventions, however, entered the world of manufacturing unheralded, such as an automated bobbin winder and a machine for testing the tensile strength of wire. He made substantial discoveries in anatomy, civil engineering, hydrodynamics, geology, optics, and tribology, but he did not publish his findings and they had little to no direct influence on subsequent science.

Cleopatra VII Thea Philopator (Koine Greek: Κλεοπάτρα Θεά Φιλοπάτωρ, lit. 'Cleopatra father-loving goddess'; 70/69 BC – 10 or 12 August 30 BC) was Queen of the Ptolemaic Kingdom of Egypt from 51 to 30 BC, and the last active Hellenistic pharaoh. A member of the Ptolemaic dynasty, she was a descendant of its founder Ptolemy I Soter, a Macedonian Greek general and companion of Alexander the Great. Her first language was Koine Greek, and she is the only Ptolemaic ruler known to have learned the Egyptian language, among several others. After her death, Egypt became a province of the Roman Empire, marking the end of the Hellenistic period in the Mediterranean, which had begun during the reign of Alexander (336–323 BC). Born in Alexandria, Cleopatra was the daughter of Ptolemy XII Auletes, who named her his heir before his death in 51 BC. Cleopatra began her reign alongside her brother Ptolemy XIII, but a falling-out between them led to a civil war. Roman statesman Pompey fled to Egypt after losing the 48 BC Battle of Pharsalus against his rival Julius Caesar, the Roman dictator, in Caesar's civil war. Pompey had been a political ally of Ptolemy XII, but Ptolemy XIII had him ambushed and killed before Caesar arrived and occupied Alexandria. Caesar then attempted to reconcile the rival Ptolemaic siblings, but Ptolemy XIII's forces besieged Cleopatra and Caesar at the palace. Shortly after reinforcements lifted the siege, Ptolemy XIII died in the Battle of the Nile. Caesar declared Cleopatra and her brother Ptolemy XIV joint rulers, and maintained a private affair with Cleopatra which produced a son, Caesarion. Cleopatra traveled to Rome as a client queen in 46 and 44 BC, where she stayed at Caesar's villa. After Caesar's assassination, followed shortly afterwards by the sudden death of Ptolemy XIV (possibly murdered on Cleopatra's order), she named Caesarion co-ruler as Ptolemy XV. In the Liberators' civil war of 43–42 BC, Cleopatra sided with the Roman Second Triumvirate formed by Caesar's heir Octavian, Mark Antony, and Marcus Aemilius Lepidus. After their meeting at Tarsos in 41 BC, the queen had an affair with Antony, which produced three children. Antony became increasingly reliant on Cleopatra for both funding and military aid during his invasions of the Parthian Empire and the Kingdom of Armenia. The Donations of Alexandria declared their children rulers over various territories under Antony's authority. Octavian portrayed this event as an act of treason, forced Antony's allies in the Roman Senate to flee Rome in 32 BC, and declared war on Cleopatra. After defeating Antony and Cleopatra's naval fleet at the 31 BC Battle of Actium, Octavian's forces invaded Egypt in 30 BC and defeated Antony, who committed suicide. After his death, Cleopatra reportedly killed herself, probably by poisoning, to avoid being publicly displayed by Octavian in a Roman triumphal procession. Cleopatra's legacy survives in ancient and modern works of art. Roman historiography and Latin poetry produced a generally critical view of the queen that pervaded later Medieval and Renaissance literature. In the visual arts, her ancient depictions include Roman busts, paintings, sculptures, cameo carvings and glass, Ptolemaic and Roman coinage, and reliefs. In Renaissance and Baroque art, she was the subject of many works including operas, paintings, poetry, sculptures, and theatrical dramas. She has become a pop culture icon of Egyptomania since the Victorian era, and in modern times has appeared in the applied and fine arts, burlesque satire, Hollywood films, and brand images for commercial products.

Martin Luther King Jr. (born Michael King Jr.; January 15, 1929 – April 4, 1968) was an American civil rights activist and Baptist minister who was a prominent leader of the civil rights movement from 1955 until his assassination in 1968. He advanced civil rights for people of color in the United States through the use of nonviolent resistance and civil disobedience against Jim Crow laws and other forms of legalized discrimination, which most commonly affected African Americans. A Black church leader, King participated in and led marches for the right to vote, desegregation, labor rights, and other civil rights. He oversaw the 1955 Montgomery bus boycott and was the first president of the Southern Christian Leadership Conference (SCLC), leading the unsuccessful Albany Movement in Albany, Georgia, and helping organize nonviolent 1963 protests in Birmingham, Alabama. King was one of the leaders of the 1963 March on Washington, where he delivered his "I Have a Dream" speech, and helped organize two of the three Selma to Montgomery marches during the 1965 Selma voting rights movement. There were dramatic standoffs with segregationist authorities, who often responded violently. The civil rights movement achieved pivotal legislative gains in the Civil Rights Act of 1964, the Voting Rights Act of 1965, and the Fair Housing Act of 1968. King was jailed several times. Federal Bureau of Investigation (FBI) director J. Edgar Hoover considered King a radical and made him an object of COINTELPRO from 1963. FBI agents investigated him for possible communist ties, spied on his personal life, and secretly recorded him. In 1964, the FBI mailed King a threatening anonymous letter, which he interpreted as an attempt to make him commit suicide. King won the 1964 Nobel Peace Prize for combating racial inequality through nonviolent resistance. In his final years, he expanded his focus to include opposition towards poverty and the Vietnam War. In 1968, King was planning a national occupation of Washington, D.C., to be called the Poor People's Campaign, when he was assassinated on April 4 in Memphis, Tennessee. James Earl Ray was convicted of the assassination, though it remains the subject of conspiracy theories. King's death led to riots in US cities. King was posthumously awarded the Presidential Medal of Freedom in 1977 and Congressional Gold Medal in 2003. Martin Luther King Jr. Day was established as a holiday in cities and states throughout the United States beginning in 1971; the federal holiday was first observed in 1986. The Martin Luther King Jr. Memorial on the National Mall in Washington, D.C., was dedicated in 2011.

Vincent Willem van Gogh (Dutch: [ˈvɪnsɛnt ˈʋɪləɱ vɑŋ ˈɣɔx] ; 30 March 1853 – 29 July 1890) was a Dutch Post-Impressionist painter who is among the most famous and influential figures in the history of Western art. In just over a decade he created about 2,100 artworks, including around 860 oil paintings, most of them in the last two years of his life. They include landscapes, still lifes, portraits and self-portraits, and are characterised by bold colours and dramatic, impulsive and expressive brushwork that contributed to the foundations of modern art. His suicide at 37 followed years of mental illness and poverty. Born into an upper-middle-class family, Van Gogh drew as a child and was serious, quiet and thoughtful. As a young man he worked as an art dealer, often travelling, but became depressed after he was transferred to London. He turned to religion, and spent time as a Protestant missionary in southern Belgium. He drifted in ill health and solitude before taking up painting in 1881, having moved back home with his parents. His younger brother Theo supported him financially, and the two kept up a long correspondence by letter. His early works, mostly still lifes and depictions of peasant labourers, contain few signs of the vivid colour that distinguished his later work. In 1886, he moved to Paris, where he met members of the avant-garde, including Émile Bernard and Paul Gauguin, who were reacting against the Impressionist sensibility. As his work developed he created a new approach to still lifes and local landscapes. His paintings grew brighter in colour as he developed a style that became fully realised during his stay in Arles in the south of France in 1888. During this period he broadened his subject matter to include series of olive trees, wheat fields and sunflowers. Van Gogh suffered from psychotic episodes and delusions and though he worried about his mental stability, he often neglected his physical health, did not eat properly and drank heavily. His friendship with Gauguin ended after a confrontation with a razor, when in a rage, he severed part of his own left ear. He spent time in psychiatric hospitals, including a period at Saint-Rémy. After he discharged himself and moved to the Auberge Ravoux in Auvers-sur-Oise near Paris, he came under the care of the homoeopathic doctor Paul Gachet. His depression continued and on 27 July 1890, Van Gogh shot himself in the chest with a revolver. He died from his injuries two days later. Van Gogh was unsuccessful during his lifetime and was considered a madman and a failure. He became famous after his suicide, and exists in the public imagination as the quintessential misunderstood genius, the artist "where discourses on madness and creativity converge". His reputation began to grow in the early 20th century as elements of his painting style came to be incorporated by the Fauves and German Expressionists. He attained widespread critical, commercial and popular success over the ensuing decades, and is remembered as an important but tragic painter, whose troubled personality typifies the romantic ideal of the tortured artist.

Wolfgang Amadeus Mozart (27 January 1756 – 5 December 1791) was a Classical composer and musician. He completed more than 800 works in his life—including outstanding examples of most of the genres of his time: symphonies, concertos, chamber music, opera, and choral music. Born in Salzburg, Mozart quickly emerged as a child prodigy under the training of his father Leopold, a skilled pedagogue. At the age of five he was already competent on keyboard and violin, had begun to compose, and had performed before European royalty. His father took him on a grand tour of Europe and then three trips to Italy. At 17 he was a musician at the Salzburg court but grew restless and travelled in search of a better position. A fruitless journey in search of employment (1777–1779) led him to Paris, Mannheim, Munich, and eventually back to Salzburg. During this time he wrote his five violin concertos, the Sinfonia Concertante, various masses, and the opera Idomeneo. While he was visiting Vienna in 1781, Mozart's quarrels with his Salzburg employers came to a head and he was dismissed. He chose to remain in Vienna, where he stayed for the rest of his life, achieving fame and some financial success, but no long-term security. During Mozart's early years in Vienna he produced several notable works, such as the opera Die Entführung aus dem Serail, the Great Mass in C minor, the "Haydn" Quartets and a number of symphonies. Throughout his Vienna years Mozart composed more than a dozen piano concertos, many considered some of his greatest achievements. In the final years of his life, he wrote many of his best-known works, including his last three symphonies, culminating in the Jupiter, the serenade Eine kleine Nachtmusik, his Clarinet Concerto, the operas Le nozze di Figaro, Don Giovanni, Così fan tutte and Die Zauberflöte, the Piano Concerto No. 27 and his Requiem. The Requiem was largely unfinished at the time of his death at the age of 35.

Alexander III of Macedon (Ancient Greek: Ἀλέξανδρος, romanized: Aléxandros; 20/21 July 356 BC – 10/11 June 323 BC), most commonly known as Alexander the Great, was king of the ancient Greek kingdom of Macedon. He succeeded his father Philip II to the throne in 336 BC at the age of 20, and spent most of his reign conducting a lengthy military campaign throughout Asia and Egypt. By the age of 30, he had created one of the largest empires in history, stretching from Greece to northwestern India. He was undefeated in battle and is widely considered to be one of history's greatest and most successful military commanders. Born in Pella, Macedon, Alexander was tutored by philosopher and polymath Aristotle until the age of 16. In 335 BC, shortly after assuming the throne of Macedon, he launched a campaign in the Balkans and reasserted control over Thrace and parts of Illyria before marching on the city of Thebes, which was subsequently destroyed. Alexander then assumed leadership of the League of Corinth, created by his father. With this authority over all Greeks he launched a pan-Hellenic invasion of the Persian Achaemenid Empire in 334 BC, and with it began a series of campaigns which lasted for ten years. Following his conquest of Asia Minor and a series of decisive battles, particularly at Issus and Gaugamela, the power of the Achaemenid Empire was broken. He subsequently overthrew Darius III and conquered the Achaemenid Empire in its entirety. After the fall of Persia, the Macedonian Empire held a vast swath of territory between the Adriatic Sea and the Indus River. Alexander endeavored to reach the "ends of the world and the Great Outer Sea" and invaded India in 326 BC, achieving an important victory over Porus, an ancient Indian king of present-day Punjab, at the Battle of the Hydaspes. Facing mutiny from troops, he was eventually forced to turn back at the Beas River, and later died in 323 BC in Babylon, Mesopotamia, the city which he had planned to make the capital of his empire, while on his return to Greece. Alexander's death put an end to his planned invasion of Arabia. In the years following his death, a series of civil wars broke out across the Macedonian Empire, eventually leading to its disintegration at the hands of the Diadochi. Alexander's death marks the conventional beginning of the Hellenistic period. Through his conquests, he built a legacy that includes the cultural diffusion and syncretism, which gave rise to Greco-Buddhism and Hellenistic Judaism. He founded more than twenty cities, with the most prominent being the city of Alexandria in Egypt. Alexander's establishment of Greek colonies and the spreading of Greek culture led to Hellenistic civilization becoming a major force in the ancient world, influencing regions as far east as the Indian subcontinent. The Hellenistic period developed through the Roman Empire into modern Western culture; the Greek language became the lingua franca of the region and was the predominant language of the Byzantine Empire until its collapse in 1453 AD. Alexander became a legendary hero similar to Achilles, featuring prominently in the historical and mythical traditions of both Greek and non-Greek cultures. His military achievements and successes in battle made him the touchstone against which many later military leaders would judge themselves. His tactics remain a significant subject of study in military academies worldwide. Legends of Alexander's exploits were coalesced into the 3rd century Alexander Romance which, in the premodern period, went through over one hundred recensions, translations, and derivations and was translated into almost every European vernacular and every language of the Islamic world. After the Bible, it was the most popular form of European literature.

Nikola Tesla (10 July 1856 – 7 January 1943) was a Serbian-American engineer, futurist, and inventor. He is known for his contributions to the design of the modern alternating current (AC) electricity supply system. Born and raised in the Austro-Hungarian Empire, Tesla first studied engineering and physics in the 1870s without receiving a degree. He then gained practical experience in the early 1880s working in telephony and at Continental Edison in the new electric power industry. In 1884, he migrated to the United States, where he became a naturalized citizen. He worked for a short time at the Edison Machine Works in New York City before he struck out on his own. With the help of partners to finance and market his ideas, Tesla set up laboratories and companies in New York to develop a range of electrical and mechanical devices. His AC induction motor and related polyphase AC patents, licensed by Westinghouse Electric in 1888, earned him a considerable amount of money and became the cornerstone of the polyphase system, which Westinghouse marketed. Tesla conducted a range of experiments with mechanical oscillators/generators, electrical discharge tubes, and early X-ray imaging among other things, in an attempt to develop inventions he could patent and market. He built a wirelessly controlled boat, one of the first wirelessly controlled vehicles ever produced. Tesla became well known as an inventor and demonstrated his achievements to celebrities and wealthy patrons at his lab. He was noted for his showmanship at public lectures. Throughout the 1890s, Tesla pursued his ideas for wireless lighting and worldwide wireless electric power distribution in his high-voltage, high-frequency power experiments in New York and Colorado Springs. In 1893, he made pronouncements on the possibility of wireless communication with his devices. Tesla tried to put these ideas to practical use in his unfinished Wardenclyffe Tower project, an intercontinental wireless communication and power transmitter, but ran out of funding before he could complete it. After Wardenclyffe, Tesla experimented with a series of inventions in the 1910s and 1920s with varying degrees of success. Having spent most of his money, Tesla lived in a series of New York hotels, leaving behind unpaid bills. He died in New York City in January 1943. Tesla's work fell into relative obscurity following his death, until 1960, when the General Conference on Weights and Measures named the International System of Units (SI) measurement of magnetic flux density the tesla in his honor. There has been a resurgence in popular interest in Tesla since the 1990s. In 2013, Time named Tesla one of the 100 most significant figures of all time.

Mohandas Karamchand Gandhi (; GAHN-dee; 2 October 1869 – 30 January 1948) was an Indian lawyer, anti-colonial nationalist and political ethicist who employed nonviolent resistance to lead the successful campaign for India's independence from British rule, and to later inspire movements for civil rights and freedom across the world. The honorific Mahātmā (Sanskrit: "great-souled", "venerable"), first applied to him in 1914 in South Africa, is now used throughout the world. Born and raised in a Hindu family in coastal Gujarat, Gandhi was trained in the law at the Inner Temple in London and was called to the bar at the age of 22. After two uncertain years in India, where he was unable to start a successful law practice, Gandhi moved to South Africa in 1893 to represent an Indian merchant in a lawsuit. He went on to live in South Africa for the next 21 years. Here, Gandhi raised a family and first employed nonviolent resistance in a campaign for civil rights. In 1915, aged 45, he returned to India and soon set about organising peasants, farmers, and urban labourers to protest against discrimination and excessive land tax. Assuming leadership of the Indian National Congress in 1921, Gandhi led nationwide campaigns for easing poverty, expanding women's rights, building religious and ethnic amity, ending untouchability, and, above all, achieving swaraj or self-rule. Gandhi adopted the short dhoti woven with hand-spun yarn as a mark of identification with India's rural poor. He began to live in a self-sufficient residential community, to eat simple food, and undertake long fasts as a means of both introspection and political protest. Bringing anti-colonial nationalism to the common Indians, Gandhi led them in challenging the British-imposed salt tax with the 400 km (250 mi) Dandi Salt March in 1930 and in calling for the British to quit India in 1942. He was imprisoned many times and for many years in both South Africa and India. Gandhi's vision of an independent India based on religious pluralism was challenged in the early 1940s by a Muslim nationalism which demanded a separate homeland for Muslims within British India. In August 1947, Britain granted independence, but the British Indian Empire was partitioned into two dominions, a Hindu-majority India and a Muslim-majority Pakistan. As many displaced Hindus, Muslims, and Sikhs made their way to their new lands, religious violence broke out, especially in Punjab and Bengal. Abstaining from the official celebration of independence, Gandhi visited the affected areas, attempting to alleviate distress. In the months following, he undertook several hunger strikes to stop the religious violence. The last of these was begun in Delhi on 12 January 1948, when Gandhi was 78. The belief that Gandhi had been too resolute in his defence of both Pakistan and Indian Muslims spread among some Hindus in India. Among these was Nathuram Godse, a militant Hindu nationalist from Pune, western India, who assassinated Gandhi by firing three bullets into his chest at an interfaith prayer meeting in Delhi on 30 January 1948. Gandhi's birthday, 2 October, is commemorated in India as Gandhi Jayanti, a national holiday, and worldwide as the International Day of Nonviolence. Gandhi is considered to be the Father of the Nation in post-colonial India. During India's nationalist movement and in several decades immediately after, he was also commonly called Bapu, an endearment roughly meaning "father".

Genghis Khan (born Temüjin; c. 1162 – August 1227), also known as Chinggis Khan, was the founder and first khan of the Mongol Empire. After spending most of his life uniting the Mongol tribes, he launched a series of military campaigns, conquering large parts of China and Central Asia. Genghis Khan was born Temüjin between 1155 and 1167 as the eldest child of Yesugei, a Mongol chieftain of the Borjigin clan, and his wife Hö'elün. When Temüjin was eight, his father died and his family was abandoned by its tribe. Reduced to near-poverty, Temüjin killed his older half-brother Behter to secure his familial position. His charismatic personality helped to attract his first followers and to form alliances with two prominent steppe leaders named Jamukha and Toghrul; they worked together to retrieve Temüjin's newlywed wife Börte, who had been kidnapped by raiders. As his reputation grew, his relationship with Jamukha deteriorated into open warfare. Temüjin was badly defeated in c. 1187, and may have spent the following years as a subject of the Jin dynasty; upon reemerging in 1196, he swiftly began gaining power. Toghrul came to view Temüjin as a threat and launched a surprise attack on him in 1203. Temüjin regrouped and overpowered Toghrul; after defeating the Naiman tribe and executing Jamukha, he was left as the sole ruler on the Mongolian steppe. Temüjin formally adopted the title "Genghis Khan", the meaning of which is uncertain, at an assembly in 1206. Carrying out reforms designed to ensure long-term stability, he transformed the Mongols' tribal structure into an integrated meritocracy dedicated to the service of the ruling family. After thwarting a coup attempt from a powerful shaman, Genghis began to consolidate his power. In 1209, he led a large-scale raid into the neighbouring Western Xia, who agreed to Mongol terms the following year. He then launched a campaign against the Jin dynasty, which lasted for four years and ended in 1215 with the capture of the Jin capital Zhongdu. His general Jebe annexed the Central Asian state of Qara Khitai in 1218. The following year, Genghis invaded the Khwarazmian Empire following the execution of his envoys; the campaign toppled the Khwarazmian state and devastated the regions of Transoxiana and Khorasan, while Jebe and his colleague Subutai led an expedition that reached Georgia and Kievan Rus'. In 1227, Genghis died while subduing the rebellious Western Xia; following a two-year interregnum, his third son and heir Ögedei acceded to the throne in 1229. Genghis Khan remains a controversial figure. He was generous and intensely loyal to his followers, but ruthless towards his enemies. He welcomed advice from diverse sources in his quest for world domination, for which he believed the shamanic supreme deity Tengri had destined him. The Mongol army under Genghis killed millions of people, yet his conquests also facilitated unprecedented commercial and cultural exchange over a vast geographical area. He is remembered as a backwards, savage tyrant in Russia and the Arab world, while recent Western scholarship has begun to reassess its previous view of him as a barbarian warlord. He was posthumously deified in Mongolia; modern Mongolians recognise him as the founding father of their nation.

Maria Salomea Skłodowska Curie (Polish: [ˈmarja salɔˈmɛa skwɔˈdɔfska kiˈri] ; née Skłodowska; 7 November 1867 – 4 July 1934), better known as Marie Curie ( KURE-ee; French: [maʁi kyʁi] ), was a Polish and naturalised-French physicist and chemist. She shared the 1903 Nobel Prize in Physics with her husband Pierre Curie "for their joint researches on the radioactivity phenomena discovered by Professor Henri Becquerel". She won the 1911 Nobel Prize in Chemistry "[for] the discovery of the elements radium and polonium, by the isolation of radium and the study of the nature and compounds of this remarkable element". She was the first woman to win a Nobel Prize, the first person to win a Nobel Prize twice, and the only person to win a Nobel Prize in two different scientific fields. Marie and Pierre were the first married couple to win the Nobel Prize and launching the Curie family legacy of five Nobel Prizes. She was, in 1906, the first woman to become a professor at the University of Paris. She was born in Warsaw, in what was then the Kingdom of Poland, part of the Russian Empire. She studied at Warsaw's clandestine Flying University and began her practical scientific training in Warsaw. In 1891, aged 24, she followed her elder sister Bronisława to study in Paris, where she earned her higher degrees and conducted her subsequent scientific work. In 1895, she married Pierre Curie, with whom she conducted pioneering research on radioactivity—a term she coined. In 1906, Pierre died in a Paris street accident. Under her direction, the world's first studies were conducted into the treatment of neoplasms by the use of radioactive isotopes. She founded the Curie Institute in Paris in 1920, and the Curie Institute in Warsaw in 1932; both remain major medical research centres. During World War I, she developed mobile radiography units to provide X-ray services to field hospitals. While a French citizen, Marie Skłodowska Curie, who used both surnames, never lost her sense of Polish identity. She taught her daughters the Polish language and took them on visits to Poland. She named the first chemical element she and Pierre discovered polonium, after her native country. Marie Curie died in 1934, aged 66, at the Sancellemoz sanatorium in Passy (Haute-Savoie), France, of aplastic anaemia likely from exposure to radiation in the course of her scientific research and in the course of her radiological work at field hospitals during World War I. In addition to her Nobel Prizes, she received numerous other honours and tributes; in 1995 she became the first woman to be entombed on her own merits in the Paris Panthéon, and Poland declared 2011 the Year of Marie Curie during the International Year of Chemistry. She is the subject of numerous biographies, including Madame Curie by her daughter Ève. The synthetic element curium is named in her honour.

Alan Mathison Turing (; 23 June 1912 – 7 June 1954) was an English mathematician, computer scientist, logician, cryptanalyst, philosopher and theoretical biologist. He was highly influential in the development of theoretical computer science, providing a formalisation of the concepts of algorithm and computation with the Turing machine, which can be considered a model of a general-purpose computer. Turing is widely considered to be the father of theoretical computer science. Born in London, Turing was raised in southern England. He graduated from King's College, Cambridge, and in 1938, earned a doctorate degree from Princeton University. During World War II, Turing worked for the Government Code and Cypher School at Bletchley Park, Britain's codebreaking centre that produced Ultra intelligence. He led Hut 8, the section responsible for German naval cryptanalysis. Turing devised techniques for speeding the breaking of German ciphers, including improvements to the pre-war Polish bomba method, an electromechanical machine that could find settings for the Enigma machine. He played a crucial role in cracking intercepted messages that enabled the Allies to defeat the Axis powers in the Battle of the Atlantic and other engagements. After the war, Turing worked at the National Physical Laboratory, where he designed the Automatic Computing Engine, one of the first designs for a stored-program computer. In 1948, Turing joined Max Newman's Computing Machine Laboratory at the University of Manchester, where he contributed to the development of early Manchester computers and became interested in mathematical biology. Turing wrote on the chemical basis of morphogenesis and predicted oscillating chemical reactions such as the Belousov–Zhabotinsky reaction, first observed in the 1960s. Despite these accomplishments, he was never fully recognised during his lifetime because much of his work was covered by the Official Secrets Act. In 1952, Turing was prosecuted for homosexual acts. He accepted hormone treatment, a procedure commonly referred to as chemical castration, as an alternative to prison. Turing died on 7 June 1954, aged 41, from cyanide poisoning. An inquest determined his death as suicide, but the evidence is also consistent with accidental poisoning. Following a campaign in 2009, British prime minister Gordon Brown made an official public apology for "the appalling way [Turing] was treated". Queen Elizabeth II granted a pardon in 2013. The term "Alan Turing law" is used informally to refer to a 2017 law in the UK that retroactively pardoned men cautioned or convicted under historical legislation that outlawed homosexual acts. Turing left an extensive legacy in mathematics and computing which has become widely recognised with statues and many things named after him, including an annual award for computing innovation. His portrait appears on the Bank of England £50 note, first released on 23 June 2021 to coincide with his birthday. The audience vote in a 2019 BBC series named Turing the greatest scientist of the 20th century. The cognitive scientist Douglas Hofstadter writes: Atheist, homosexual, eccentric, marathon-running mathematician, A. M. Turing was in large part responsible not only for the concept of computers, incisive theorems about their powers, and a clear vision of the possibility of computer minds, but also for the cracking of German ciphers during the Second World War. It is fair to say we owe much to Alan Turing for the fact that we are not under Nazi rule today."

Gaius Julius Caesar (12 or 13 July 100 BC – 15 March 44 BC) was a Roman general, statesman, and author who was the dictator of the Roman Republic almost continuously from 49 BC until his assassination in 44 BC. A member of the First Triumvirate, he led the Roman armies through the Gallic Wars and defeated his political rival Pompey in Caesar's civil war. He consolidated power and proclaimed himself dictator for life in 44 BC, helping create the political conditions that led to the collapse of the Roman Republic and the emergence of the Roman Empire. For his role in these events, he is regarded as one of history’s most influential figures. In 60 BC, Caesar, Crassus, and Pompey formed the First Triumvirate, an informal political alliance that dominated Roman politics for several years. Their attempts to amass political power were opposed by many in the Senate, including Cato the Younger with the private support of Cicero. Caesar rose to become one of the most powerful politicians in the Roman Republic through a string of military victories in the Gallic Wars, completed by 51 BC, which greatly expanded Roman territory. During this time, he invaded Britain and built a bridge across the Rhine. These achievements, along with the loyalty of his veteran army, threatened to eclipse the standing of Pompey. The alliance between Caesar and Pompey slowly broke down and, by 50 BC, Pompey had realigned himself with the Senate. With his command expiring and the Gallic Wars largely concluded, the Senate ordered Caesar to relinquish his military command and return to Rome. In early January 49 BC, Caesar openly defied the Senate by crossing the Rubicon and marching towards Rome at the head of an army. This act began Caesar's civil war, which he won, leaving him in a position of near-unchallenged power and influence in 45 BC. After assuming control of government and pardoning many of his enemies, Caesar carried out various reforms and public works. He created the Julian calendar to replace the republican lunisolar calendar, reduced the size of the grain dole, settled his veterans in new overseas colonies, greatly increased the size of the Senate, and extended citizenship to communities in Spain and what is now northern Italy. In early 44 BC, he was proclaimed "dictator for life". Fearful of his power, his domination of the state, and the possibility that he might make himself king, a group of senators led by Brutus and Cassius assassinated Caesar on the Ides of March (15 March) 44 BC. A new series of civil wars broke out and the constitutional government of the Republic was never fully restored. Caesar's great-nephew and adoptive heir Octavian, later known as Augustus, rose to sole power after defeating his opponents thirteen years later. Octavian then set about consolidating his power, transforming the Republic into the Roman Empire as its first emperor. Caesar was an accomplished author and commentator; much of his life is known through his own accounts of his military campaigns. Other contemporary sources include the letters and speeches of Cicero and the historical writings of Sallust. Later biographies of Caesar by Suetonius and Plutarch are also important sources. Caesar is considered by many historians to be one of the greatest military commanders in history. His cognomen subsequently became a synonym for emperor; the title Caesar was used throughout the Roman Empire, and gave rise to modern derivatives such as Kaiser and Tsar. He has frequently appeared in literary and artistic works.

Ludwig van Beethoven (baptised 17 December 1770 – 26 March 1827) was a German composer, conductor, and pianist. Mentored during the Classical period, Beethoven's musical style was a key driver of the transition to Romantic music, and the expansion of popular forms such as the symphony and string quartet. His compositions have attracted casual and scholarly interest, and remain among the most performed in the world. Born in Bonn, Beethoven was a musical prodigy. He was initially taught intensively by his father, Johann van Beethoven, and later by Christian Gottlob Neefe. He found relief from a dysfunctional home life with the family of Helene von Breuning, whose children he loved, befriended, and taught piano. At age 21, he moved to Vienna, which subsequently became his base, and studied composition with Joseph Haydn. Beethoven gained a reputation as a virtuoso pianist, and was soon patronized by Karl Alois, Prince Lichnowsky for compositions, which resulted in his three Piano Trios, Op. 1, (the earliest works to which he accorded an opus number) in 1795. His Pathétique Sonata was composed in 1798. Around this time, Beethoven began experiencing symptoms of hearing loss. His career is divided into three periods. In the first, he composed in the classical style of Haydn and Wolfgang Amadeus Mozart. Beethoven's First Symphony premiered in 1800, and his first set of string quartets was published in 1801. The Moonlight Sonata, dedicated to his pupil Julie Guicciardi, is one of his most popular works. In the middle period (1802–1812), he developed a distinctive style. His Third (Eroica) and Fifth Symphonies premiered in 1805 and 1808, respectively. The former was unprecedented in scale and scope. His opera, Fidelio, premiered in 1805, and was later revised. His Violin Concerto appeared in 1806. His Razumovsky quartets were published in 1808. Other middle works include the Waldstein and Appassionata sonatas for piano, the Kreutzer sonata for violin and his Seventh Symphony. His Emperor piano concerto premiered in 1811, without the composer as soloist. His late period (1812–1827) included some of his most innovative works. His Missa solemnis and his final symphony, the Ninth, premiered in 1824. The latter was the first major example of a choral symphony. His Diabelli Variations and late piano sonatas, particularly the Hammerklavier, are summits of the keyboard literature. His late string quartets, including the Große Fuge, of 1825–1826, are considered pinnacles of the genre. After several months of illness, which left him bedridden, Beethoven died on 26 March 1827 at the age of 56.

Sir Isaac Newton ( ; (1643-01-04)4 January 1643 [O.S. 25 December 1642] – 31 March [O.S. 20 March] 1727) was an English polymath who was a mathematician, physicist, astronomer, alchemist, theologian, author and inventor. He was a key figure in the Scientific Revolution and the Enlightenment that followed. His book Philosophiæ Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy), first published in 1687, achieved the first great unification in physics and established classical mechanics. Newton also made seminal contributions to optics, and shares credit with the German mathematician Gottfried Wilhelm Leibniz for formulating infinitesimal calculus, although he developed calculus years before Leibniz. Newton contributed to and refined the scientific method, and his work is considered the most influential in bringing forth modern science. In the Principia, Newton formulated the laws of motion and universal gravitation that formed the dominant scientific viewpoint for centuries until it was superseded by the theory of relativity. While this is the case, his laws still serve as excellent approximations for the vast majority of physical phenomena involving low speeds (much less than the speed of light) and weak gravitational fields. He used his mathematical description of gravity to derive Kepler's laws of planetary motion, account for tides, the trajectories of comets, the precession of the equinoxes and other phenomena, eradicating doubt about the Solar System's heliocentricity. Newton solved the two-body problem and introduced the three-body problem. He demonstrated that the motion of objects on Earth and celestial bodies could be accounted for by the same principles. Newton's inference that the Earth is an oblate spheroid was later confirmed by the geodetic measurements of Alexis Clairaut, Charles Marie de La Condamine, and others, convincing most European scientists of the superiority of Newtonian mechanics over earlier systems. He was also the first to calculate the age of Earth by experiment, and described a precursor to the modern wind tunnel. Further, he was the first to provide a quantitative estimate of the solar mass. Newton built the first reflecting telescope and developed a sophisticated theory of colour based on the observation that a prism separates white light into the colours of the visible spectrum. His work on light was collected in his book Opticks, published in 1704. He originated prisms as beam expanders and multiple-prism arrays, which would later become integral to the development of tunable lasers. Newton invented a double-reflecting quadrant and was the first to theorise the Goos–Hänchen effect. He also formulated an empirical law of cooling, which was the first heat transfer formulation and serves as the formal basis of convective heat transfer, made the first theoretical calculation of the speed of sound, and introduced the notions of a Newtonian fluid and a black body. He was also the first to explain the Magnus effect. Moreover, he was the first to analyse Couette flow. In addition to his creation of calculus, Newton's work on mathematics was extensive. He generalised the binomial theorem to any real number, introduced the Puiseux series, was the first to state Bézout's theorem, classified most of the cubic plane curves, contributed to the study of Cremona transformations, developed a method for approximating the roots of a function, originated the Newton–Cotes formulas used for numerical integration, and further produced the earliest explicit enunciation of the general Taylor series. Additionally, Newton initiated the field of calculus of variations, formulated and solved the earliest problem in geometric probability, devised the earliest form of linear regression, and was a pioneer of vector analysis. Newton was a fellow of Trinity College and the second Lucasian Professor of Mathematics at the University of Cambridge; he was appointed at the age of 26. He was a devout but unorthodox Christian who privately rejected the doctrine of the Trinity. He refused to take holy orders in the Church of England, unlike most members of the Cambridge faculty of the day. Beyond his work on the mathematical sciences, Newton dedicated much of his time to the study of alchemy and biblical chronology, but most of his work in those areas remained unpublished until long after his death. Politically and personally tied to the Whigs, Newton served two brief terms as Member of Parliament for the University of Cambridge, in 1689–1690 and 1701–1702. He was knighted by Queen Anne in 1705 and spent the last three decades of his life in London, serving as Warden (1696–1699) and Master (1699–1727) of the Royal Mint, in which he increased the accuracy and security of British coinage. He was also the president of the Royal Society (1703–1727).

Abraham Lincoln (February 12, 1809 – April 15, 1865) was the 16th president of the United States, serving from 1861 until his assassination in 1865. He led the United States through the American Civil War, defeating the Confederacy and playing a major role in the abolition of slavery. Born in a one-room log cabin in Kentucky, Lincoln was raised on the frontier. He was self-educated and became a lawyer, Illinois state legislator, and U.S. representative. Angered by the Kansas–Nebraska Act of 1854, which opened the territories to slavery, he became a leader of the new Republican Party. He reached a national audience in the 1858 Senate campaign debates against Stephen A. Douglas. Lincoln won the 1860 presidential election, becoming the first Republican president. His victory prompted a majority of the slave states to begin seceding and form the Confederate States. A month after Lincoln assumed the presidency, Confederate forces attacked Fort Sumter, starting the Civil War. As a moderate Republican, Lincoln had to navigate conflicting political opinions from contentious factions during the war effort. He closely supervised the Union's strategy and tactics, including the selection of generals and the implementation of a naval blockade of Southern ports. He suspended the writ of habeas corpus in April 1861, an action that Chief Justice Roger Taney found in Ex parte Merryman that only Congress could do, and he averted war with Britain by defusing the Trent Affair. On January 1, 1863, he issued the Emancipation Proclamation, which declared the slaves in the states "in rebellion" to be free. On November 19, 1863, he delivered the Gettysburg Address, which became one of the most famous speeches in American history. He promoted the Thirteenth Amendment to the U.S. Constitution, which, in 1865, abolished chattel slavery. Following his re-election in 1864, he sought to heal the war-torn nation through Reconstruction. On April 14, 1865, five days after the Confederate surrender at Appomattox, Lincoln was fatally shot by John Wilkes Booth at Ford's Theatre in Washington, D.C., becoming the first U.S. president to be assassinated. Lincoln is remembered as a martyr and a national hero for his wartime leadership and for his efforts to preserve the Union and abolish slavery. He is consistently ranked in both popular and scholarly polls as among the greatest presidents in American history.

Philosophy of mind is a branch of philosophy that deals with the nature of the mind and its relation to the body and the external world. The mind–body problem is a paradigmatic issue in philosophy of mind, although a number of other issues are addressed, such as the hard problem of consciousness and the nature of particular mental states. Aspects of the mind that are studied include mental events, mental functions, mental properties, consciousness and its neural correlates, the ontology of the mind, the nature of cognition and of thought, and the relationship of the mind to the body. Dualism and monism are the two central schools of thought on the mind–body problem, although nuanced views have arisen that do not fit either category neatly. Dualism finds its entry into Western philosophy thanks to René Descartes in the 17th century. Substance dualists like Descartes argue that the mind is an independently existing substance, whereas property dualists maintain that the mind is a group of independent properties that emerge from and cannot be reduced to the brain, but that it is not a distinct substance. Monism is the position that mind and body are ontologically indiscernible entities, not dependent substances. This view was espoused by the 17th-century rationalist Baruch Spinoza. Physicalists argue that only entities postulated by physical theory exist, and that mental processes will eventually be explained in terms of these entities as physical theory continues to evolve. Physicalists maintain various positions on the prospects of reducing mental properties to physical properties (many of whom adopt compatible forms of property dualism), and the ontological status of such mental properties remains unclear. Idealists maintain that the mind is all that exists and that the external world is either mental itself, or an illusion created by the mind. Neutral monists such as Ernst Mach and William James argue that events in the world can be thought of as either mental (psychological) or physical depending on the network of relationships into which they enter, and dual-aspect monists such as Spinoza adhere to the position that there is some other, neutral substance, and that both matter and mind are properties of this unknown substance. The most common monisms in the 20th and 21st centuries have all been variations of physicalism; these positions include behaviorism, type physicalism, anomalous monism and functionalism. Most modern philosophers of mind adopt either a reductive physicalist or non-reductive physicalist position, maintaining in their different ways that the mind is not something separate from the body. These approaches have been particularly influential in the sciences, especially in the fields of sociobiology, computer science (specifically, artificial intelligence), evolutionary psychology and the various neurosciences. Reductive physicalists assert that all mental states and properties will eventually be explained by scientific accounts of physiological processes and states. Non-reductive physicalists argue that although the mind is not a separate substance, mental properties supervene on physical properties, or that the predicates and vocabulary used in mental descriptions and explanations are indispensable, and cannot be reduced to the language and lower-level explanations of physical science. Continued neuroscientific progress has helped to clarify some of these issues; however, they are far from being resolved. Modern philosophers of mind continue to ask how the subjective qualities and the intentionality of mental states and properties can be explained in naturalistic terms.

Consciousness is being aware of something internal to one's self, or of states or objects in one's external environment. It has been the topic of extensive explanations, analyses, and debate among philosophers, scientists, and theologians for millennia. There is no consensus on what exactly needs to be studied, or whether consciousness can be considered a scientific concept. In some explanations it is synonymous with mind, while in others it is considered an aspect of it. In the past, consciousness meant one's "inner life": the world of introspection, private thought, imagination, and volition. Today, it often includes various forms of cognition, experience, feeling, or perception. It may be awareness, awareness of awareness, metacognition, or self-awareness, either continuously changing or not. There is also a medical definition that helps, for example, to discern "coma" from other states. The disparate range of research, notions, and speculations raises some curiosity about whether the right questions are being asked. Examples of the range of descriptions, definitions and explanations are: ordered distinction between self and environment, simple wakefulness, one's sense of selfhood or soul explored by "looking within", being a metaphorical "stream" of contents, or being a mental state, mental event, or mental process of the brain.

Jean-Paul Charles Aymard Sartre (, US also ; French: [saʁtʁ]; 21 June 1905 – 15 April 1980) was a French philosopher, playwright, novelist, screenwriter, political activist, biographer, and literary critic, considered a leading figure in 20th-century French philosophy and Marxism. Sartre was one of the key figures in the philosophy of existentialism (and phenomenology). His work has influenced sociology, critical theory, post-colonial theory, and literary studies. He was awarded the 1964 Nobel Prize in Literature despite attempting to refuse it, saying that he always declined official honors and that "a writer should not allow himself to be turned into an institution." Sartre had an open relationship with prominent feminist and fellow existentialist philosopher Simone de Beauvoir. Together, Sartre and de Beauvoir challenged the cultural and social assumptions and expectations of their upbringings, which they considered bourgeois, in both lifestyles and thought. The conflict between oppressive, spiritually destructive conformity (mauvaise foi, literally, 'bad faith') and an "authentic" way of "being" became the dominant theme of Sartre's early work, a theme embodied in his principal philosophical work Being and Nothingness (L'Être et le Néant, 1943). Sartre provided an introduction to his philosophy in his work Existentialism Is a Humanism (L'existentialisme est un humanisme, 1946), originally presented as a lecture. Born in Paris, Sartre lost his father at age two and was raised primarily by his mother and grandfather, who introduced him to literature. He studied at the prestigious École Normale Supérieure, where he developed a deep interest in philosophy, influenced by thinkers like Henri Bergson, Edmund Husserl, and Martin Heidegger. Sartre's early academic career included teaching in several French lycées and engaging in provocative pranks and debates. Sartre's life was marked by strong political engagement. During World War II, he was drafted, captured, and later released, after which he co-founded the resistance group Socialisme et Liberté. Though the group dissolved, Sartre became an influential voice in occupied France, contributing to underground literature and writing plays like No Exit. After the war, he co-founded the journal Les Temps modernes and increasingly used his platform to advocate for political and social causes. He supported anti-colonial movements, condemned French policies in Algeria, opposed U.S. intervention in Vietnam, and aligned himself at various times with Marxism and Maoism. Despite declining health in his later years, Sartre remained committed to activism and intellectual debate until his death in 1980. His funeral drew 50,000 mourners.

Friedrich Wilhelm Nietzsche (15 October 1844 – 25 August 1900) was a German philosopher and writer who started his career as a classical philologist and turned to philosophy early in his academic career. In 1869, at age 24, he was appointed Professor of Classical Philology at the University of Basel. Plagued by health problems for most of his life, he resigned from the university in 1879. He afterward lived as an independent writer, spending much of his life in relative solitude and financial insecurity while moving between Switzerland, Italy, and southern France in search of climates that might alleviate his condition, and in the following decade, he completed much of his core writing. In 1889, aged 44, he suffered a neurological collapse, and thereafter a complete loss of his mental faculties, with paralysis and vascular dementia; he lived out his remaining 11 years, first under the care of his mother, and later his sister, until his death. His works and his philosophy have fostered not only extensive scholarship but also much popular interest. Nietzsche's work encompasses poetry, cultural criticism, and philosophical essays while displaying a fondness for aphorisms and irony. Prominent elements of his philosophy include his critique of objective truth in favour of perspectivism; a genealogical critique of Christian morality and a related theory of master–slave morality; the affirmation of life in response to both the weakening of religion ("God is dead") and the crisis of passive nihilism which followed it; the notion of Apollonian and Dionysian duality; and a characterisation of the human subject as the expression of competing wills, collectively understood as the will-to-power. His later work developed influential concepts such as the Übermensch, the transvaluation of values, and his unique formulation of eternal return. His body of work touched a wide range of topics, including aesthetic, philology, history, music, religion, tragedy, culture and science, and drew inspiration from such diverse sources as Greek tragedy, Arthur Schopenhauer, Ralph Waldo Emerson, and Richard Wagner. After Nietzsche's death, his sister, Elisabeth Förster-Nietzsche, became the curator and editor of his manuscripts. She edited his unpublished writings to fit her German ultranationalist ideology, often contradicting or obfuscating Nietzsche's stated opinions, which were explicitly opposed to antisemitism and nationalism. Through her published editions, Nietzsche's work became associated with fascism and Nazism. Twentieth-century scholars such as Walter Kaufmann, R. J. Hollingdale and Georges Bataille defended Nietzsche against this interpretation, and corrected editions of his writings were soon made available. The works of Nietzsche then achieved renewed popularity in the 1960s and his ideas have since had a widespread impact on 20th- and 21st-century thinkers across philosophy—especially in schools of continental philosophy such as existentialism, postmodernism and post-structuralism—as well as art, literature, music, poetry, politics and popular culture.

Epistemology is the branch of philosophy that examines the nature, origin, and limits of knowledge. Also called the theory of knowledge, it explores different types of knowledge, such as propositional knowledge about facts, practical knowledge in the form of skills, and knowledge by acquaintance as a familiarity through experience. Epistemologists study the concepts of belief, truth, and justification to understand the nature of knowledge. To discover how knowledge arises, they investigate sources of justification, such as perception, introspection, memory, reason, and testimony. The school of skepticism questions the human ability to attain knowledge, while fallibilism says that knowledge is never certain. Empiricists hold that all knowledge comes from sense experience, whereas rationalists believe that some knowledge does not depend on it. Coherentists argue that a belief is justified if it is consistent with other beliefs. Foundationalists, by contrast, maintain that the justification of basic beliefs does not depend on other beliefs. Internalism and externalism debate whether justification is determined solely by mental states or also by external circumstances. Separate branches of epistemology focus on knowledge in specific fields, like scientific, mathematical, moral, and religious knowledge. Naturalized epistemology relies on empirical methods and discoveries, whereas formal epistemology uses formal tools from logic. Social epistemology investigates the communal aspect of knowledge, and historical epistemology examines its historical conditions. Epistemology is closely related to psychology, which infers the beliefs people hold from their words and actions, while epistemology studies the norms governing the evaluation of beliefs. It also intersects with fields such as decision theory, education, and anthropology. Early reflections on the nature, sources, and scope of knowledge are found in ancient Greek, Indian, and Chinese philosophy. The relation between reason and faith was a central topic in the medieval period. The modern era was characterized by the contrasting perspectives of empiricism and rationalism. Epistemologists in the 20th century examined the components, structure, and value of knowledge while integrating insights from the natural sciences and linguistics.

Baruch (de) Spinoza (24 November 1632 – 21 February 1677), also known under his Latinized pen name Benedictus de Spinoza, was a philosopher of Portuguese-Jewish origin, who was born and lived in the Dutch Republic. A forerunner of the Enlightenment, Spinoza significantly influenced modern biblical criticism, 17th-century rationalism, and Dutch intellectual culture, establishing himself as one of the most important and radical philosophers of the early modern period. Influenced by Stoicism, Thomas Hobbes, René Descartes, and heterodox Christians, Spinoza was a leading philosopher of the Dutch Golden Age. Spinoza was born in Amsterdam to a Marrano family that fled Portugal for the more tolerant Dutch Republic. He received a traditional Jewish education, learning Hebrew and studying sacred texts within the Portuguese Jewish community, where his father was a prominent merchant. As a young man, Spinoza challenged rabbinic authority and questioned Jewish doctrines, leading to his permanent expulsion from his Jewish community in 1656. Following that expulsion, he distanced himself from all religious affiliations and devoted himself to philosophical inquiry, supporting himself by lens grinding. Spinoza attracted a dedicated circle of followers who gathered to discuss his writings and joined him in the intellectual pursuit of truth. Spinoza published sparingly during his lifetime to circumvent persecution and the suppression of his works. In his Tractatus Theologico-Politicus, described by Steven Nadler as "one of the most important books of Western thought", Spinoza questioned the divine origin of the Hebrew Bible and the nature of God while arguing that ecclesiastic authority should have no role in a secular, democratic state. Ethics equates God with nature, arguing for a view of God that scholars interpret variously as pantheistic, panentheistic, or atheistic, and it explores the place of human freedom in a world devoid of theological, cosmological, and political moorings. Rejecting messianism and the emphasis on the afterlife, Spinoza emphasized appreciating and valuing life for oneself and others. By advocating for individual liberty in its moral, psychological, and metaphysical dimensions, Spinoza helped establish the genre of political writing called secular theology. Spinoza’s philosophy addresses multiple areas of philosophical discourse, including metaphysics, epistemology, political philosophy, ethics, philosophy of mind, and philosophy of science. His works were published posthumously by his friends and influenced philosophers for the next two centuries. Spinoza is celebrated as one of the most original and influential thinkers of the seventeenth century. Rebecca Goldstein described him as "the renegade Jew who gave us modernity".

In philosophy, empiricism is an epistemological view which holds that true knowledge or justification comes either only or primarily from sensory experience and empirical evidence. It is one of several competing views within epistemology, along with rationalism and skepticism. Empiricists argue that empiricism is a more reliable method of finding the truth than relying purely on logical reasoning, because humans have cognitive biases and limitations which lead to errors of judgement. Empiricism emphasizes the central role of empirical evidence in the formation of ideas, rather than innate ideas or traditions. Empiricists may argue that traditions (or customs) arise due to relations of previous sensory experiences. Historically, empiricism was associated with the "blank slate" concept (tabula rasa), according to which the human mind is "blank" at birth and develops its thoughts only through later experience. Empiricism in the philosophy of science emphasizes evidence, especially as discovered in experiments. It is a fundamental part of the scientific method that all hypotheses and theories must be tested against observations of the natural world rather than resting solely on a priori reasoning, intuition, or revelation. Empiricism, often used by natural scientists, holds that "knowledge is based on experience" and that "knowledge is tentative and probabilistic, subject to continued revision and falsification". Empirical research, including experiments and validated measurement tools, guides the scientific method.

Existentialism is a family of philosophical views and inquiry that explore the human individual's struggle to lead an authentic life despite the apparent absurdity or incomprehensibility of existence. In examining meaning, purpose, and value, existentialist thought often includes concepts such as existential crises, angst, courage, and freedom. Existentialism is associated with several 19th- and 20th-century European philosophers who shared an emphasis on the human subject, despite often profound differences in thought. Among the 19th-century figures now associated with existentialism are philosophers Søren Kierkegaard and Friedrich Nietzsche, as well as novelist Fyodor Dostoevsky, all of whom critiqued rationalism and concerned themselves with the problem of meaning. The word existentialism, however, was not coined until the mid 20th century, during which it became most associated with contemporaneous philosophers Jean-Paul Sartre, Martin Heidegger, Simone de Beauvoir, Karl Jaspers, Gabriel Marcel, Paul Tillich, and more controversially Albert Camus. Many existentialists considered traditional systematic or academic philosophies, in style and content, to be too abstract and removed from concrete human experience. A primary virtue in existentialist thought is authenticity. Existentialism would influence many disciplines outside of philosophy, including theology, drama, art, literature, and psychology. Existentialist philosophy encompasses a range of perspectives, but it shares certain underlying concepts. Among these, a central tenet of existentialism is that personal freedom, individual responsibility, and deliberate choice are essential to the pursuit of self-discovery and the determination of life's meaning.

Metaphysics is the branch of philosophy that examines the basic structure of reality. It is traditionally seen as the study of mind-independent features of the world, but some theorists view it as an inquiry into the conceptual framework of human understanding. Some philosophers, including Aristotle, designate metaphysics as the first philosophy to suggest that it is more fundamental than other forms of philosophical inquiry. Metaphysics encompasses a wide range of general and abstract topics. It investigates the nature of existence, the features all entities have in common, and their division into categories of being. An influential division is between particulars and universals. Particulars are unique individual entities, like a specific apple. Universals are general features that different particulars have in common, like the color red. Modal metaphysics examines what it means for something to be possible or necessary. Metaphysicians also explore the concepts of space, time, and change, and their connection to causality and the laws of nature. Other topics include how mind and matter are related, whether everything in the world is predetermined, and whether there is free will. Metaphysicians use various methods to conduct their inquiry. They typically rely on rational intuitions and abstract reasoning but also include empirical approaches associated with scientific theories. Due to the abstract nature of its topic, metaphysics has received criticisms questioning the reliability of its methods and the meaningfulness of its theories. Metaphysics is relevant to many fields of inquiry that often implicitly rely on metaphysical concepts and assumptions. The roots of metaphysics lie in antiquity with speculations about the nature and origin of the universe, like those found in the Upanishads in ancient India, Taoism in ancient China, and pre-Socratic philosophy in ancient Greece. During the subsequent medieval period in the West, discussions about the nature of universals were influenced by the philosophies of Plato and Aristotle. The modern period saw the emergence of various comprehensive systems of metaphysics, many of which embraced idealism. In the 20th century, traditional metaphysics in general and idealism in particular faced various criticisms, which prompted new approaches to metaphysical inquiry.

Ethics is the philosophical study of moral phenomena. Also called moral philosophy, it investigates normative questions about what people ought to do or which behavior is morally right. Its main branches include normative ethics, applied ethics, and metaethics. Normative ethics aims to find general principles that govern how people should act. Applied ethics examines concrete ethical problems in real-life situations, such as abortion, treatment of animals, and business practices. Metaethics explores the underlying assumptions and concepts of ethics. It asks whether there are objective moral facts, how moral knowledge is possible, and how moral judgments motivate people. Influential normative theories are consequentialism, deontology, and virtue ethics. According to consequentialists, an act is right if it leads to the best consequences. Deontologists focus on acts themselves, saying that they must adhere to duties, like telling the truth and keeping promises. Virtue ethics sees the manifestation of virtues, like courage and compassion, as the fundamental principle of morality. Ethics is closely connected to value theory, which studies the nature and types of value, like the contrast between intrinsic and instrumental value. Moral psychology is a related empirical field and investigates psychological processes involved in morality, such as reasoning and the formation of character. Descriptive ethics describes the dominant moral codes and beliefs in different societies and considers their historical dimension. The history of ethics started in the ancient period with the development of ethical principles and theories in ancient Egypt, India, China, and Greece. This period saw the emergence of ethical teachings associated with Hinduism, Buddhism, Confucianism, Daoism, and contributions of philosophers like Socrates and Aristotle. During the medieval period, ethical thought was strongly influenced by religious teachings. In the modern period, this focus shifted to a more secular approach concerned with moral experience, reasons for acting, and the consequences of actions. An influential development in the 20th century was the emergence of metaethics.

Nihilism is a family of philosophical views that reject the existence of any objectively meaningful purpose, moral value, truth, or knowledge. Nihilistic views span several branches of philosophy, including ethics, value theory, epistemology, and metaphysics. Nihilism is also described as a broad cultural phenomenon or historical movement that pervades modernity in the Western world. Existential nihilism asserts that life has no objective meaning or purpose. The idea that all individual and societal values are ultimately pointless has been associated with various responses. They range from general indifference and existential crises to transformative reinterpretations of established ideals and a creative embrace of personal meaning-making. Moral nihilism, a related view, denies the objective existence of morality, arguing that moral evaluations and practices rest on misguided assumptions without any foundation in external reality. In epistemology or the theory of knowledge, nihilism challenges knowledge and truth. According to relativism, knowledge, truth, or meaning are relative to the perspectives of specific individuals or cultural contexts. This implies there is no independent framework to assess which opinion is ultimately correct. Skeptical interpretations go further by denying the existence of knowledge or truth altogether. In metaphysics, one form of nihilism states that the universe could have been empty without any objects. This view holds that there is no fundamental reason for why something exists rather than nothing. Mereological nihilism asserts there are only simple objects, like elementary particles, but no composite objects, like tables. Cosmological nihilism is the view that reality is unintelligible and indifferent to human understanding. Other nihilist positions include political, semantic, logical, and therapeutic nihilism. Some aspects of nihilism have their roots in ancient philosophy in the form of challenges to established beliefs, values, and practices. However, nihilism is primarily associated with modernity, emerging in the 18th and 19th centuries, particularly in Germany and Russia through the works of Friedrich Heinrich Jacobi and Ivan Turgenev. It took center stage in the thought of Friedrich Nietzsche, who understood nihilism as a pervasive cultural trend in which people lose the traditional values and ideals guiding their lives as a result of secularization. In the 20th century, nihilist themes were explored by Dadaism, existentialism, and postmodern philosophy.

Democracy is a form of government in which political power is vested in the people or the population of a state. Under a minimalist definition of democracy, rulers are elected through competitive elections while more expansive or maximalist definitions link democracy to guarantees of civil liberties and human rights in addition to competitive elections. In a direct democracy, the people have the direct authority to deliberate and decide legislation. In a representative democracy, the people choose governing officials through elections to do so. The definition of "the people" and the ways authority is shared among them or delegated by them have changed over time and at varying rates in different countries. Features of democracy often include freedom of assembly, association, personal property, freedom of religion and speech, citizenship, consent of the governed, voting rights, freedom from unwarranted governmental deprivation of the right to life and liberty, and minority rights. The notion of democracy has evolved considerably over time. Throughout history, one can find evidence of direct democracy, in which communities make decisions through popular assembly. Today, the dominant form of democracy is representative democracy, where citizens elect government officials to govern on their behalf such as in a parliamentary or presidential democracy. In the common variant of liberal democracy, the powers of the majority are exercised within the framework of a representative democracy, but a constitution and supreme court limit the majority and protect the minority—usually through securing the enjoyment by all of certain individual rights, such as freedom of speech or freedom of association. The term appeared in the 5th century BC in Greek city-states, notably Classical Athens, to mean "rule of the people", in contrast to aristocracy (ἀριστοκρατία, aristokratía), meaning "rule of an elite". In virtually all democratic governments throughout ancient and modern history, democratic citizenship was initially restricted to an elite class, which was later extended to all adult citizens. In most modern democracies, this was achieved through the suffrage movements of the 19th and 20th centuries. Democracy contrasts with forms of government where power is not vested in the general population of a state, such as authoritarian systems. Historically a rare and vulnerable form of government, democratic systems of government have become more prevalent since the 19th century, in particular with various waves of democratization. Democracy garners considerable legitimacy in the modern world, as public opinion across regions tends to strongly favor democratic systems of government relative to alternatives, and as even authoritarian states try to present themselves as democratic. Democracy more consistently results in improved health, education and economic outcomes. According to the V-Dem Democracy indices and The Economist Democracy Index, less than half the world's population lives in a democracy as of 2022. At the same time, while representative democracy is widely valued, Pew Research Center found that dissatisfaction with democratic performance is common even in many established democracies.

David Hume (; born David Home; 7 May 1711 – 25 August 1776) was a Scottish philosopher, historian, economist and essayist who is known for his highly influential system of empiricism, philosophical scepticism and metaphysical naturalism. Beginning with A Treatise of Human Nature (1739–40), Hume strove to create a naturalistic science of man that examined the psychological basis of human nature. Hume followed John Locke in rejecting the existence of innate ideas, concluding that all human knowledge derives solely from experience; this places him amongst such empiricists as Francis Bacon, Thomas Hobbes, Locke and George Berkeley. Hume argued that inductive reasoning and belief in causality cannot be justified empirically; instead, they result from custom and mental habit. People never actually perceive that one event causes another but experience only the "constant conjunction" of events. This problem of induction means that to draw any causal inferences from past experience, it is necessary to presuppose that the future will resemble the past; this metaphysical presupposition cannot itself be grounded in prior experience. An opponent of philosophical rationalists, Hume held that passions rather than reason govern human behaviour, proclaiming that "Reason is, and ought only to be the slave of the passions." Hume was also a sentimentalist who held that ethics are based on emotion or sentiment rather than abstract moral principle. He maintained an early commitment to naturalistic explanations of moral phenomena and is usually accepted by historians of European philosophy to have first clearly expounded the is–ought problem, or the idea that a statement of fact alone can never give rise to a normative conclusion of what ought to be done. Hume denied that people have an actual conception of the self, positing that they experience only a bundle of sensations and that the self is nothing more than this bundle of perceptions connected by an association of ideas. Hume's compatibilist theory of free will takes causal determinism as fully compatible with human freedom. His philosophy of religion, including his rejection of miracles and critique of the argument from design, was especially controversial. Hume left a legacy that affected utilitarianism, logical positivism, the philosophy of science, early analytic philosophy, cognitive science, theology and many other fields and thinkers. Immanuel Kant credited Hume as the inspiration that had awakened him from his "dogmatic slumbers".

Simone Lucie Ernestine Marie Bertrand de Beauvoir (9 January 1908 – 14 April 1986) was a French existentialist philosopher, writer, social theorist, and feminist activist. Though she did not consider herself a philosopher, nor was she considered one at the time of her death, she had a significant influence on both feminist existentialism and feminist theory. Beauvoir wrote novels, essays, short stories, biographies, autobiographies, and monographs on philosophy, politics, and social issues. She was best known for her "trailblazing work in feminist philosophy", The Second Sex (1949), a detailed analysis of women's oppression and a foundational tract of contemporary feminism. She was also known for her novels, the most famous of which were She Came to Stay (1943) and The Mandarins (1954). Her most enduring contribution to literature is her memoirs, notably the first volume, Mémoires d'une jeune fille rangée (1958). She received the 1954 Prix Goncourt, the 1975 Jerusalem Prize, and the 1978 Austrian State Prize for European Literature. She was also nominated for the Nobel Prize in Literature in 1961, 1969 and 1973. However, Beauvoir generated controversy when she briefly lost her teaching job after being accused of sexually abusing some of her students.

Thomas Hobbes ( HOBZ; 5 April 1588 – 4 December 1679) was an English philosopher and political theorist, best known for his 1651 book Leviathan, in which he expounds an influential formulation of social contract theory. He is considered to be one of the founders of modern political philosophy. In his early life, overshadowed by his father's departure following a fight, he was taken under the care of his wealthy uncle. Hobbes's academic journey began in Westport, leading him to the University of Oxford, where he was exposed to classical literature and mathematics. He then graduated from the University of Cambridge in 1608. He became a tutor to the Cavendish family, which connected him to intellectual circles and initiated his extensive travels across Europe. These experiences, including meetings with figures like Galileo, shaped his intellectual development. After returning to England from France in 1637, Hobbes witnessed the destruction and brutality of the English Civil War from 1642 to 1651 between Parliamentarians and Royalists, which heavily influenced his advocacy for governance by an absolute sovereign in Leviathan, as the solution to human conflict and societal breakdown. Aside from social contract theory, Leviathan also popularised ideas such as the state of nature ("war of all against all") and laws of nature. His other major works include the trilogy De Cive (1642), De Corpore (1655), and De Homine (1658) as well as the posthumous work Behemoth (1681). Hobbes contributed to a diverse array of fields, including history, jurisprudence, geometry, optics, theology, classical translations, ethics, as well as philosophy in general, marking him as a polymath. Despite controversies and challenges to his reputation, including accusations of atheism and contentious debates with contemporaries, Hobbes's work profoundly influenced the understanding of political structure and human nature.

Free will is generally understood as the capacity or ability of people to (a) choose between different possible courses of action, (b) exercise control over their actions in a way that is necessary for moral responsibility, or (c) be the ultimate source or originator of their actions. There are different theories as to its nature, and these aspects are often emphasized differently depending on philosophical tradition, with debates focusing on whether and how such freedom can coexist with physical determinism, divine foreknowledge, and other constraints. Free will is closely linked to the concepts of moral responsibility and moral desert, praise, culpability, and other judgements that can logically apply only to actions that are freely chosen. It is also connected with the concepts of advice, persuasion, deliberation, and prohibition. Traditionally, only actions that are freely willed are seen as deserving credit or blame. Whether free will exists and the implications of whether it exists or not constitute some of the longest-running debates of philosophy. Some philosophers and thinkers conceive of free will as the capacity to make choices undetermined by past events. However, determinism suggests that the natural world is governed by cause-and-effect relationships, and only one course of events is possible, which is inconsistent with a libertarian model of free will. Ancient Greek philosophy identified this issue, which remains a major focus of philosophical debate to this day. The view that posits free will as incompatible with determinism is called incompatibilism and encompasses both metaphysical libertarianism (the claim that determinism is false and thus free will is at least possible) and hard determinism or hard incompatibilism (the claim that determinism is true and thus free will is not possible). Another incompatibilist position is illusionism or hard incompatibilism, which holds not only determinism but also indeterminism (randomness) to be incompatible with free will and thus free will to be impossible regardless of the metaphysical truth of determinism. In contrast, compatibilists hold that free will is compatible with determinism. Some compatibilist philosophers (i.e., hard compatibilists) even hold that determinism is actually necessary for the existence of free will and agency, on the grounds that choice involves preference for one course of action over another, requiring a sense of how choices will turn out. Different compatibilists offer very different definitions of what "free will" means and consequently find different types of constraints to be relevant to the issue. Classical compatibilists considered free will nothing more than freedom of action, considering one free of will if, had one counterfactually wanted to do otherwise, one could have done otherwise without physical impediment. Many contemporary compatibilists instead identify free will as a psychological capacity, such as to direct one's behavior in a way that is responsive to reason or potentially sanctionable. There are still further different conceptions of free will, each with its own concerns, sharing only the common feature of not finding the possibility of physical determinism a threat to the possibility of free will.

Phenomenology is a philosophical study and movement largely associated with the early 20th century that seeks to objectively investigate the nature of subjective, conscious experience. It attempts to describe the universal features of consciousness while avoiding assumptions about the external world, aiming to describe phenomena as they appear, and to explore the meaning and significance of lived experience. This approach, while philosophical, has found many applications in qualitative research across different scientific disciplines, especially in the social sciences, humanities, psychology, and cognitive science, but also in fields as diverse as health sciences, architecture, and human-computer interaction, among many others. The application of phenomenology in these fields aims to gain a deeper understanding of subjective experience, rather than focusing on behavior. Phenomenology is contrasted with phenomenalism, which reduces mental states and physical objects to complexes of sensations, and with psychologism, which treats logical truths or epistemological principles as the products of human psychology. In particular, transcendental phenomenology, as outlined by Edmund Husserl, aims to arrive at an objective understanding of the world via the discovery of universal logical structures in human subjective experience. There are important differences in the ways that different branches of phenomenology approach subjectivity. For example, according to Martin Heidegger, truths are contextually situated and dependent on the historical, cultural, and social context in which they emerge. Other types include hermeneutic, genetic, and embodied phenomenology. All these different branches of phenomenology may be seen as representing different philosophies despite sharing the common foundational approach of phenomenological inquiry; that is, investigating things just as they appear, independent of any particular theoretical framework.

John Stuart Mill (20 May 1806 – 7 May 1873) was an English philosopher, political economist, politician and civil servant. One of the most influential thinkers in the history of liberalism and social liberalism, he contributed widely to social theory, political theory, and political economy. Dubbed "the most influential English-speaking philosopher of the nineteenth century" by the Stanford Encyclopedia of Philosophy, he conceived of liberty as justifying the freedom of the individual in opposition to unlimited state and social control. He advocated political and social reforms such as proportional representation, the emancipation of women, and the development of labour organisations and farm cooperatives. The Columbia Encyclopedia describes Mill as occasionally coming "close to socialism, a theory repugnant to his predecessors". He was a proponent of utilitarianism, an ethical theory developed by his predecessor Jeremy Bentham. He contributed to the investigation of scientific methodology, though his knowledge of the topic was based on the writings of others, notably William Whewell, John Herschel, and Auguste Comte, and research carried out for Mill by Alexander Bain. He engaged in a written debate with Whewell. A member of the Liberal Party and co-author of the early feminist work The Subjection of Women (1869), Mill was also the second Member of Parliament to call for women's suffrage after Henry Hunt in 1832. The ideas presented in his essay On Liberty (1859) have remained the basis of much political thought, and a copy is passed to the president of the Liberal Democrats (the successor party to Mill's own) as a symbol of office.

Logic is the study of correct reasoning. It includes both formal and informal logic. Formal logic is the study of deductively valid inferences or logical truths. It examines how conclusions follow from premises based on the structure of arguments alone, independent of their topic and content. Informal logic is associated with informal fallacies, critical thinking, and argumentation theory. Informal logic examines arguments expressed in natural language whereas formal logic uses formal language. When used as a countable noun, the term "a logic" refers to a specific logical formal system that articulates a proof system. Logic plays a central role in many fields, such as philosophy, mathematics, computer science, and linguistics. Logic studies arguments, which consist of a set of premises that leads to a conclusion. An example is the argument from the premises "it's Sunday" and "if it's Sunday then I don't have to work" leading to the conclusion "I don't have to work." Premises and conclusions express propositions or claims that can be true or false. An important feature of propositions is their internal structure. For example, complex propositions are made up of simpler propositions linked by logical vocabulary like ∧ {\displaystyle \land } (and) or → {\displaystyle \to } (if...then). Simple propositions also have parts, like "Sunday" or "work" in the example. The truth of a proposition usually depends on the meanings of all of its parts. However, this is not the case for logically true propositions. They are true only because of their logical structure independent of the specific meanings of the individual parts. Arguments can be either correct or incorrect. An argument is correct if its premises support its conclusion. Deductive arguments have the strongest form of support: if their premises are true then their conclusion must also be true. This is not the case for ampliative arguments, which arrive at genuinely new information not found in the premises. Many arguments in everyday discourse and the sciences are ampliative arguments. They are divided into inductive and abductive arguments. Inductive arguments are statistical generalizations, such as inferring that all ravens are black based on many individual observations of black ravens. Abductive arguments are inferences to the best explanation, for example, when a doctor concludes that a patient has a certain disease which explains the symptoms they suffer. Arguments that fall short of the standards of correct reasoning often embody fallacies. Systems of logic are theoretical frameworks for assessing the correctness of arguments. Logic has been studied since antiquity. Early approaches include Aristotelian logic, Stoic logic, Nyaya, and Mohism. Aristotelian logic focuses on reasoning in the form of syllogisms. It was considered the main system of logic in the Western world until it was replaced by modern formal logic, which has its roots in the work of late 19th-century mathematicians such as Gottlob Frege. Today, the most commonly used system is classical logic. It consists of propositional logic and first-order logic. Propositional logic only considers logical relations between full propositions. First-order logic also takes the internal parts of propositions into account, like predicates and quantifiers. Extended logics accept the basic intuitions behind classical logic and apply it to other fields, such as metaphysics, ethics, and epistemology, as frameworks for reasoning about what is possible or necessary, what is or should be, and what is believed or known. Deviant logics, on the other hand, reject certain classical intuitions and provide alternative explanations of the basic laws of logic.

Karl Marx (German: [ˈkaʁl ˈmaʁks]; 5 May 1818 – 14 March 1883) was a German philosopher, social and political theorist, economist, journalist, and revolutionary socialist. He developed the theory of historical materialism, analyzing class struggle under capitalism and predicting the system's overthrow by the proletariat in favour of communism. Marx co-authored The Communist Manifesto (1848) with his lifelong friend Friedrich Engels, and undertook a critique of classical political economy in his magnum opus, Das Kapital (1867–1894). Marx's ideas and their subsequent development, collectively known as Marxism, have had enormous influence and have influenced revolutions and uprisings in many countries. Born in Trier in the Kingdom of Prussia, Marx earned a doctoral degree in philosophy from the University of Jena in 1841. A Young Hegelian, he was influenced by the philosophy of Georg Wilhelm Friedrich Hegel, and both critiqued and developed Hegel's ideas in works such as The German Ideology (written 1846) and the Grundrisse (written 1857–1858). While in Paris, Marx wrote his Economic and Philosophic Manuscripts of 1844 and met Engels, who became a close collaborator. After moving to Brussels in 1845, they were active in the Communist League, and in 1848 wrote The Communist Manifesto, which expresses Marx's ideas and lays out a programme for revolution. Marx was expelled from Belgium and Germany, and in 1849 moved to London, where he wrote The Eighteenth Brumaire of Louis Bonaparte (1852) and Das Kapital. From 1864, Marx was involved in the International Workingmen's Association (First International), in which he fought the influence of anarchists led by Mikhail Bakunin. In his Critique of the Gotha Programme (1875), Marx wrote on revolution, the state and the transition to communism. He died stateless in 1883 and was buried in Highgate Cemetery. Marx's critiques of history, society and political economy hold that human societies develop through class conflict. In the capitalist mode of production, this manifests itself in the conflict between the ruling classes (the bourgeoisie) that control the means of production and the working classes (the proletariat) that enable these means by selling their labour power for wages. Employing his historical materialist approach, Marx predicted that capitalism produced internal contradictions like previous socioeconomic systems, and that these tensions would lead to its self-destruction and replacement by a new system. For Marx, class antagonisms under capitalism—owing in part to its instability and crisis-prone nature—would eventuate the working class's development of class consciousness, leading to their conquest of political power and eventually the establishment of a classless, communist society constituted by a free association of producers. Marx actively pressed for its implementation, arguing that the working class should carry out organised proletarian revolutionary action to topple capitalism and bring about socio-economic emancipation. Marx has been described as one of the most influential figures of the modern era, and his work has been both lauded and criticised. Marxism has exerted major influence on socialist thought and political movements, with Marxist schools of thought such as Marxism–Leninism and its offshoots becoming the guiding ideologies of revolutions that took power in many countries during the 20th century, forming communist states. Marx's work in economics has had a strong influence on modern heterodox theories of labour and capital, and he is often cited as one of the principal architects of modern sociology.

Aristotle (Ancient Greek: Ἀριστοτέλης, romanized: Aristotélēs; 384–322 BC) was an ancient Greek philosopher and polymath. His writings span the natural sciences, philosophy, linguistics, economics, politics, psychology, and the arts. As the founder of the Peripatetic school of philosophy in the Lyceum in Athens, he began the wider Aristotelian tradition that followed, which set the groundwork for the development of modern science. Little is known about Aristotle's life. He was born in the city of Stagira in Chalkidiki (northern Greece) during the Classical period. His father, Nicomachus, died when Aristotle was a child, and he was brought up by a guardian. At around eighteen years old, he joined Plato's Academy in Athens and remained there until the age of thirty seven (c. 347 BC). Shortly after Plato died, Aristotle left Athens and, at the request of Philip II of Macedon, tutored his son Alexander the Great beginning in 343 BC. He established a library in the Lyceum, which helped him to produce many of his hundreds of books on papyrus scrolls. Though Aristotle wrote many treatises and dialogues for publication, only around a third of his original output has survived, none of it intended for publication. Aristotle provided a complex synthesis of the various philosophies existing prior to him. His teachings and methods of inquiry have had a significant impact across the world, and remain a subject of contemporary philosophical discussion. Aristotle's views profoundly shaped medieval scholarship. The influence of his physical science extended from late antiquity and the Early Middle Ages into the Renaissance, and was not replaced systematically until the Enlightenment and theories such as classical mechanics were developed. He influenced Judeo-Islamic philosophies during the Middle Ages, as well as Christian theology, especially the Neoplatonism of the Early Church and the scholastic tradition of the Catholic Church. Aristotle was revered among medieval Muslim scholars as "The First Teacher", and among medieval Christians like Thomas Aquinas as simply "The Philosopher". He has been referred to as the first scientist. His works contain the earliest known systematic study of logic, and were studied by medieval scholars such as Peter Abelard and Jean Buridan. His influence on logic continued well into the 19th century. In addition, his ethics, although always influential, has gained renewed interest with the modern advent of virtue ethics.

Immanuel Kant (born Emanuel Kant; 22 April 1724 – 12 February 1804) was a German philosopher. Born in Königsberg in the Kingdom of Prussia, he is considered one of the central thinkers of the Enlightenment. His comprehensive and systematic works in epistemology, metaphysics, logic, ethics, aesthetics, political theory, and the philosophy of religion have made him one of the most influential and highly discussed figures in modern Western philosophy. Kant's philosophy is centered on the human subject and motivated by the desire to secure the possibility of both knowledge and morality against the threats of skepticism and determinism. In the Critique of Pure Reason (1781/1787), Kant argues for transcendental idealism, the doctrine that space and time are mere "forms of intuition" (German: Anschauung) that structure all experience and that we have knowledge only of "appearances" and not of the nature of things in themselves. Kant drew a parallel to the Copernican Revolution in his proposal to think of the objects of experience as conforming to people's spatial and temporal forms of intuition and the categories of the understanding, instead of the traditional method of showing how the mind might conform to its objects. Kant believed that reason is the source of morality and that the categorical imperative binds all rational agents. He believed that aesthetics arises from a faculty of disinterested judgment. Kant hoped that perpetual peace could be secured through an international federation of republican states and international cooperation. Kant believed that true religion is grounded on morality. The exact nature of his religious views is a matter of dispute.

John Locke (; 29 August 1632 (O.S.) – 28 October 1704 (O.S.)) was an English philosopher and physician, widely regarded as one of the most influential of the Enlightenment thinkers and commonly known as the "father of liberalism". His important works include A Letter Concerning Toleration (1689), Two Treatises of Government (1689/90), both published anonymously, and An Essay Concerning Human Understanding (1689/90). His writing on toleration contends that religion is a matter for the individual and that the churches are voluntary associations, ruling out religious coercion and uniformity; these lead to the idea of separation of church and state. His Two Treatises on Government argues for government based on the consent of the governed and the right to revolt against tyrannous government which has lost consent. The Two Treatises are believed to have influenced the language that Thomas Jefferson chose in his drafting the July 1776 Declaration of Independence during the War of American Independence. Locke lived through the tumultuous political era of the English Civil War and Commonwealth of England after the execution of Charles I, Restoration of the Stuart monarchy, and the 1688 Glorious Revolution. These experiences affected his political thinking and life choices. During the Interregnum, Locke won a place at Christ Church, Oxford after attending the prestigious Westminster School. He spent 15 years at Oxford, first as a student, then as a tutor, pursuing medical and other scientific interests in a circle of friends. In 1666, Locke became an associate of Lord Shaftesbury, a key figure in English political life after the Restoration (1660), and Locke was appointed to governmental posts at Shaftesbury's recommendation. Locke was elected a Fellow of the Royal Society (1668). When Shaftesbury fell from royal favour and died shortly thereafter, Locke went into political exile for five years in the Netherlands (1683–85). There he wrote some of his most important works. During this period, he gained the patronage of Thomas Herbert, 8th Earl of Pembroke. Locke returned to England from exile, accompanying Queen Mary II in 1689. He published three of his most notable works soon after his return. He served as a Commissioner of Trade and Plantations, then retired from public life due to ill health. For the last fourteen years of his life, he lived in the household of Sir Francis Masham and his wife, philosopher Lady Masham, whom Locke had known since she was a young woman. Considered one of the first of the British empiricists, following the tradition of Francis Bacon, Locke is equally important to social contract theory. His work greatly affected the development of epistemology and political philosophy. His writings influenced Voltaire, Jean-Jacques Rousseau, and many Scottish Enlightenment thinkers, as well as the American Revolutionaries. His contributions to classical republicanism and liberal theory are reflected in the United States Declaration of Independence. Internationally, Locke's political-legal principles continue to have a profound influence on the theory and practice of limited representative government and the protection of basic rights and freedoms under the rule of law. Locke's philosophy of mind is often cited as the origin of modern conceptions of personal identity and the psychology of self, figuring prominently in the work of later philosophers, such as Rousseau, David Hume, and Immanuel Kant. He postulated that, at birth, the mind is a blank slate, or tabula rasa. Contrary to Cartesian philosophy based on preexisting concepts, he maintained that we are born without innate ideas, and that knowledge is instead determined only by experience derived from sense perception, a concept now known as empiricism. Locke is often credited for describing private property as a natural right, arguing that when a person—metaphorically—mixes their labour with nature, resources can be removed from the common state of nature.

Plato ( PLAY-toh; Ancient Greek: Πλάτων, Plátōn; born c. 428–423 BC, died 348/347 BC) was an ancient Greek philosopher of Classical Athens who is most commonly considered the foundational thinker of the Western philosophical tradition. An innovator of the literary dialogue and dialectic forms, Plato influenced all the major areas of theoretical philosophy and practical philosophy, and was the founder of the Academy, a philosophical school in Athens where Plato taught the collection of philosophical theories that would later become known as Platonism. Plato's most famous contribution is his Theory of Forms (or Ideas), which aims to solve what is now known as the problem of universals. He was influenced by the pre-Socratic thinkers Pythagoras, Heraclitus, and Parmenides, although much of what is known about them is derived from Plato himself. Along with his teacher Socrates, and his student Aristotle, Plato is a central figure in the history of Western philosophy. Plato's complete works are believed to have survived for over 2,400 years—unlike that of nearly all of his contemporaries. Although their popularity has fluctuated, they have consistently been read and studied through the ages. Through Platonism's outgrowth Neoplatonism, he also influenced Christian, Jewish and Islamic philosophy. In modern times, Alfred North Whitehead said: "the safest general characterization of the European philosophical tradition is that it consists of a series of footnotes to Plato."

Pragmatism is a philosophical tradition that views language and thought as tools for prediction, problem solving, and action, rather than describing, representing, or mirroring reality. Pragmatists contend that most philosophical topics—such as the nature of knowledge, language, concepts, meaning, belief, and science—are best viewed in terms of their practical uses and successes. Pragmatism began in the United States in the 1870s. Its origins are often attributed to philosophers Charles Sanders Peirce, William James and John Dewey. In 1878, Peirce described it in his pragmatic maxim: "Consider the practical effects of the objects of your conception. Then, your conception of those effects is the whole of your conception of the object."

Magnetism is the class of physical attributes that occur through a magnetic field, which allows objects to attract or repel each other. Because both electric currents and magnetic moments of elementary particles give rise to a magnetic field, magnetism is one of two aspects of electromagnetism. The most familiar effects occur in ferromagnetic materials, which are strongly attracted by magnetic fields and can be magnetized to become permanent magnets, producing magnetic fields themselves. Demagnetizing a magnet is also possible. Only a few substances are ferromagnetic; the most common ones are iron, cobalt, nickel, and their alloys. All substances exhibit some type of magnetism. Magnetic materials are classified according to their bulk susceptibility. Ferromagnetism is responsible for most of the effects of magnetism encountered in everyday life, but there are actually several types of magnetism. Paramagnetic substances, such as aluminium and oxygen, are weakly attracted to an applied magnetic field; diamagnetic substances, such as copper and carbon, are weakly repelled; while antiferromagnetic materials, such as chromium, have a more complex relationship with a magnetic field. The force of a magnet on paramagnetic, diamagnetic, and antiferromagnetic materials is usually too weak to be felt and can be detected only by laboratory instruments, so in everyday life, these substances are often described as non-magnetic. The strength of a magnetic field always decreases with distance from the magnetic source, though the exact mathematical relationship between strength and distance varies. Many factors can influence the magnetic field of an object including the magnetic moment of the material, the physical shape of the object, both the magnitude and direction of any electric current present within the object, and the temperature of the object.

In physics, electromagnetism is an interaction that occurs between particles with electric charge via electromagnetic fields. The electromagnetic force is one of the four fundamental forces of nature. It is the dominant force in the interactions of atoms and molecules. Electromagnetism can be thought of as a combination of electrostatics and magnetism, which are distinct but closely intertwined phenomena. Electromagnetic forces occur between any two charged particles. Electric forces cause an attraction between particles with opposite charges and repulsion between particles with the same charge, while magnetism is an interaction that occurs between charged particles in relative motion. These two forces are described in terms of electromagnetic fields. Macroscopic charged objects are described in terms of Coulomb's law for electricity and Ampère's force law for magnetism; the Lorentz force describes microscopic charged particles. The electromagnetic force is responsible for many of the chemical and physical phenomena observed in daily life. The electrostatic attraction between atomic nuclei and their electrons holds atoms together. Electric forces also allow different atoms to combine into molecules, including the macromolecules such as proteins that form the basis of life. Meanwhile, magnetic interactions between the spin and angular momentum magnetic moments of electrons also play a role in chemical reactivity; such relationships are studied in spin chemistry. Electromagnetism also plays several crucial roles in modern technology: electrical energy production, transformation and distribution; light, heat, and sound production and detection; fiber optic and wireless communication; sensors; computation; electrolysis; electroplating; and mechanical motors and actuators. Electromagnetism has been studied since ancient times. Many ancient civilizations, including the Greeks and the Mayans, created wide-ranging theories to explain lightning, static electricity, and the attraction between magnetized pieces of iron ore. However, it was not until the late 18th century that scientists began to develop a mathematical basis for understanding the nature of electromagnetic interactions. In the 18th and 19th centuries, prominent scientists and mathematicians such as Coulomb, Gauss and Faraday developed namesake laws which helped to explain the formation and interaction of electromagnetic fields. This process culminated in the 1860s with the discovery of Maxwell's equations, a set of four partial differential equations which provide a complete description of classical electromagnetic fields. Maxwell's equations provided a sound mathematical basis for the relationships between electricity and magnetism that scientists had been exploring for centuries, and predicted the existence of self-sustaining electromagnetic waves. Maxwell postulated that such waves make up visible light, which was later shown to be true. Gamma-rays, x-rays, ultraviolet, visible, infrared radiation, microwaves and radio waves were all determined to be electromagnetic radiation differing only in their range of frequencies. In the modern era, scientists continue to refine the theory of electromagnetism to account for the effects of modern physics, including quantum mechanics and relativity. The theoretical implications of electromagnetism, particularly the requirement that observations remain consistent when viewed from various moving frames of reference (relativistic electromagnetism) and the establishment of the speed of light based on properties of the medium of propagation (permeability and permittivity), helped inspire Einstein's theory of special relativity in 1905. Quantum electrodynamics (QED) modifies Maxwell's equations to be consistent with the quantized nature of matter. In QED, changes in the electromagnetic field are expressed in terms of discrete excitations, particles known as photons, the quanta of light.

In theoretical physics, quantum field theory (QFT) is a theoretical framework that combines field theory, special relativity and quantum mechanics. QFT is used in particle physics to construct physical models of subatomic particles and in condensed matter physics to construct models of quasiparticles. The current Standard Model of particle physics is based on QFT. Despite its extraordinary predictive success, QFT faces ongoing challenges in fully incorporating gravity and in establishing a completely rigorous mathematical foundation.

Optics is the branch of physics that studies the behaviour, manipulation, and detection of electromagnetic radiation, including its interactions with matter and instruments that use or detect it. Optics usually describes the behaviour of visible, ultraviolet, and infrared light. The study of optics extends to other forms of electromagnetic radiation, including radio waves, microwaves, and X-rays. The term optics is also applied to technology for manipulating beams of elementary charged particles. Most optical phenomena can be accounted for by using the classical electromagnetic description of light, however, complete electromagnetic descriptions of light are often difficult to apply in practice. Practical optics is usually done using simplified models. The most common of these, geometric optics, treats light as a collection of rays that travel in straight lines and bend when they pass through or reflect from surfaces. Physical optics is a more comprehensive model of light, which includes wave effects such as diffraction and interference that cannot be accounted for in geometric optics. Historically, the ray-based model of light was developed first, followed by the wave model of light. Progress in electromagnetic theory in the 19th century led to the discovery that light waves were in fact electromagnetic radiation. Some phenomena depend on light having both wave-like and particle-like properties. Explanation of these effects requires quantum mechanics. When considering light's particle-like properties, the light is modelled as a collection of particles called "photons". Quantum optics deals with the application of quantum mechanics to optical systems. Optical science is relevant to and studied in many related disciplines including astronomy, various engineering fields, photography, and medicine, especially in radiographic methods such as beam radiation therapy and CT scans, and in the physiological optical fields of ophthalmology and optometry. Practical applications of optics are found in a variety of technologies and everyday objects, including mirrors, lenses, telescopes, microscopes, lasers, and fibre optics.

The atomic nucleus is the small, dense region consisting of protons and neutrons at the center of an atom, discovered in 1911 by Ernest Rutherford at the University of Manchester based on the 1909 Geiger–Marsden gold foil experiment. After the discovery of the neutron in 1932, models for a nucleus composed of protons and neutrons were quickly developed by Dmitri Ivanenko and Werner Heisenberg. An atom is composed of a positively charged nucleus, with a cloud of negatively charged electrons surrounding it, bound together by electrostatic force. Almost all of the mass of an atom is located in the nucleus, with a very small contribution from the electron cloud. Protons and neutrons are bound together to form a nucleus by the nuclear force. The diameter of the nucleus is in the range of 1.70 fm (1.70×10−15 m) for hydrogen (the diameter of a single proton) to about 11.7 fm for uranium. These dimensions are much smaller than the diameter of the atom itself (nucleus + electron cloud), by a factor of about 27,000 (uranium's atomic radius is about 156 pm (156×10−12 m)) to about 60,000 (hydrogen's atomic radius is about 52.92 pm). The branch of physics involved with the study and understanding of the atomic nucleus, including its composition and the forces that bind it together, is called nuclear physics.

The photoelectric effect is the emission of electrons from a material caused by electromagnetic radiation such as ultraviolet light. Electrons emitted in this manner are called photoelectrons. The phenomenon is studied in condensed matter physics, solid state, and quantum chemistry to draw inferences about the properties of atoms, molecules and solids. The effect has found use in electronic devices specialized for light detection and precisely timed electron emission. The experimental results disagree with classical electromagnetism, which predicts that continuous light waves transfer energy to electrons, which would then be emitted when they accumulate enough energy. An alteration in the intensity of light would theoretically change the kinetic energy of the emitted electrons, with sufficiently dim light resulting in a delayed emission. The experimental results instead show that electrons are dislodged only when the light exceeds a certain frequency—regardless of the light's intensity or duration of exposure. Because a low-frequency beam at a high intensity does not build up the energy required to produce photoelectrons, as would be the case if light's energy accumulated over time from a continuous wave, Albert Einstein proposed that a beam of light is not a wave propagating through space, but discrete energy packets, which were later popularised as photons by Gilbert N. Lewis. Emission of conduction electrons from typical metals requires a few electron-volt (eV) light quanta, corresponding to short-wavelength visible or ultraviolet light. In extreme cases, emissions are induced with photons approaching zero energy, like in systems with negative electron affinity and the emission from excited states, or a few hundred keV photons for core electrons in elements with a high atomic number. Study of the photoelectric effect led to important steps in understanding the quantum nature of light and electrons and influenced the formation of the concept of wave–particle duality. Other phenomena where light affects the movement of electric charges include the photoconductive effect, the photovoltaic effect, and the photoelectrochemical effect.

Newton's laws of motion are three physical laws that describe the relationship between the motion of an object and the forces acting on it. These laws, which provide the basis for Newtonian mechanics, can be paraphrased as follows: A body remains at rest, or in motion at a constant speed in a straight line, unless it is acted upon by a force. At any instant of time, the net force on a body is equal to the body's acceleration multiplied by its mass or, equivalently, the rate at which the body's momentum is changing with time. If two bodies exert forces on each other, these forces have the same magnitude but opposite directions. The three laws of motion were first stated by Isaac Newton in his Philosophiæ Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy), originally published in 1687. Newton used them to investigate and explain the motion of many physical objects and systems. In the time since Newton, new insights, especially around the concept of energy, built the field of classical mechanics on his foundations. In modern times, limitations to Newton's laws have been discovered; new theories were consequently developed, such as quantum mechanics and relativity to address the physics of objects in more extreme cases.

A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word laser originated as an acronym for light amplification by stimulated emission of radiation. The first laser was built in 1960 by Theodore Maiman at Hughes Research Laboratories, based on theoretical work by Charles H. Townes and Arthur Leonard Schawlow and the optical amplifier patented by Gordon Gould. A laser differs from other sources of light in that it emits light that is coherent. Spatial coherence allows a laser to be focused to a tight spot, enabling uses such as optical communication, laser cutting, and lithography. It also allows a laser beam to stay narrow over great distances (collimation), used in laser pointers, lidar, and free-space optical communication. Lasers can also have high temporal coherence, which permits them to emit light with a very narrow frequency spectrum. Temporal coherence can also be used to produce ultrashort pulses of light with a broad spectrum but durations measured in attoseconds. Lasers are used in fiber-optic and free-space optical communications, optical disc drives, laser printers, barcode scanners, semiconductor chip manufacturing (photolithography, etching), laser surgery and skin treatments, cutting and welding materials, military and law enforcement devices for marking targets and measuring range and speed, and in laser lighting displays for entertainment. The laser is regarded as one of the greatest inventions of the 20th century.

In astronomy and cosmology, dark matter is an invisible and hypothetical form of matter that does not interact with electromagnetic radiation, including light. Dark matter is implied by gravitational effects that cannot be explained by general relativity unless more matter is present than can be observed. Such effects occur in the context of formation and evolution of galaxies, gravitational lensing, the observable universe's current structure, mass position in galactic collisions, the motion of galaxies within galaxy clusters, and cosmic microwave background anisotropies. Dark matter is thought to serve as gravitational scaffolding for cosmic structures. After the Big Bang, dark matter clumped into blobs along narrow filaments with superclusters of galaxies forming a cosmic web at scales on which entire galaxies appear like tiny particles. In the standard Lambda-CDM model of cosmology, the mass–energy content of the universe is 5% ordinary matter, 26.8% dark matter, and 68.2% a form of energy known as dark energy. Thus, dark matter constitutes 85% of the total mass, while dark energy and dark matter constitute 95% of the total mass–energy content. While the density of dark matter is significant in the halo around a galaxy, its local density in the Solar System is much less than normal matter. The total of all the dark matter out to the orbit of Neptune would add up to about 1017 kg, the same as a large asteroid. Dark matter is classified as "cold", "warm", or "hot" according to velocity (more precisely, its free streaming length). Recent models have favored a cold dark matter scenario, in which structures emerge by the gradual accumulation of particles. Dark matter is not known to interact with ordinary baryonic matter and radiation except through gravity, making it difficult to detect in the laboratory. The most prevalent explanation is that dark matter is some as-yet-undiscovered subatomic particle, such as either weakly interacting massive particles (WIMPs) or axions. The other main possibility is that dark matter is composed of primordial black holes. Although the astrophysics community generally accepts the existence of dark matter, a minority of astrophysicists, intrigued by specific observations that are not well explained by ordinary dark matter, argue for various modifications of the standard laws of general relativity. These include modified Newtonian dynamics (MOND), tensor–vector–scalar gravity, and entropic gravity. So far, none of the proposed modified gravity theories can describe every piece of observational evidence at the same time, suggesting that even if gravity has to be modified, some form of dark matter would still be required.

Nuclear fission is a reaction in which the nucleus of an atom splits into two or more smaller nuclei. The fission process often produces gamma photons, and releases a very large amount of energy even by the energetic standards of radioactive decay. Nuclear fission was discovered by chemists Otto Hahn and Fritz Strassmann and physicists Lise Meitner and Otto Robert Frisch. Hahn and Strassmann proved that a fission reaction had taken place on 19 December 1938, and Meitner and her nephew Frisch explained it theoretically in January 1939. Frisch named the process "fission" by analogy with biological fission of living cells; according to Richard Rhodes, the idea arose in a discussion with physicist William A. Arnold, who suggested "binary fission". In their second publication on nuclear fission in February 1939, Hahn and Strassmann predicted the existence and liberation of additional neutrons during the fission process, opening up the possibility of a nuclear chain reaction. For heavy nuclides, it is an exothermic reaction which releases large amounts of energy both as electromagnetic radiation and as kinetic energy of the fragments (heating the bulk material where fission takes place). Like nuclear fusion, for fission to produce energy, the total binding energy of the resulting elements must be greater than that of the starting element. The fission barrier must also be overcome. Fissionable nuclides primarily split in interactions with fast neutrons, while fissile nuclides easily split in interactions with "slow" i.e. thermal neutrons, usually originating from moderation of fast neutrons. Fission is a form of nuclear transmutation because the resulting fragments (or daughter atoms) are not the same element as the original parent atom. The two (or more) nuclei produced are most often of comparable but slightly different sizes, typically with a mass ratio of products of about 3 to 2, for common fissile isotopes. Most fissions are binary fissions (producing two charged fragments), but occasionally (2 to 4 times per 1000 events), three positively charged fragments are produced, in a ternary fission. The smallest of these fragments in ternary processes ranges in size from a proton to an argon nucleus. Apart from fission induced by an exogenous neutron, harnessed and exploited by humans, a natural form of spontaneous radioactive decay (not requiring an exogenous neutron, because the nucleus already has an overabundance of neutrons) is also referred to as fission, and occurs especially in very high-mass-number isotopes. Spontaneous fission was discovered in 1940 by Flyorov, Petrzhak, and Kurchatov in Moscow. In contrast to nuclear fusion, which drives the formation of stars and their development, one can consider nuclear fission as negligible for the evolution of the universe. Nonetheless, natural nuclear fission reactors may form under very rare conditions. Accordingly, all elements (with a few exceptions, see "spontaneous fission") which are important for the formation of solar systems, planets and also for all forms of life are not fission products, but rather the results of fusion processes. The unpredictable composition of the products (which vary in a broad probabilistic and somewhat chaotic manner) distinguishes fission from purely quantum tunneling processes such as proton emission, alpha decay, and cluster decay, which give the same products each time. Nuclear fission produces energy for nuclear power and drives the explosion of nuclear weapons. Both uses are possible because certain substances called nuclear fuels undergo fission when struck by fission neutrons, and in turn emit neutrons when they break apart. This makes a self-sustaining nuclear chain reaction possible, releasing energy at a controlled rate in a nuclear reactor or at a very rapid, uncontrolled rate in a nuclear weapon. The amount of free energy released in the fission of an equivalent amount of 235U is a million times more than that released in the combustion of methane or from hydrogen fuel cells. The products of nuclear fission, however, are on average far more radioactive than the heavy elements which are normally fissioned as fuel, and remain so for significant amounts of time, giving rise to a nuclear waste problem. However, the seven long-lived fission products make up only a small fraction of fission products. Neutron absorption which does not lead to fission produces plutonium (from 238U) and minor actinides (from both 235U and 238U) whose radiotoxicity is far higher than that of the long lived fission products. Concerns over nuclear waste accumulation and the destructive potential of nuclear weapons are a counterbalance to the peaceful desire to use fission as an energy source. The thorium fuel cycle produces virtually no plutonium and much less minor actinides, but 232U - or rather its decay products - are a major gamma ray emitter. All actinides are fertile or fissile and fast breeder reactors can fission them all albeit only in certain configurations. Nuclear reprocessing aims to recover usable material from spent nuclear fuel to both enable uranium (and thorium) supplies to last longer and to reduce the amount of "waste". The industry term for a process that fissions all or nearly all actinides is a "closed fuel cycle".

General relativity, also known as the general theory of relativity, and as Einstein's theory of gravity, is the geometric theory of gravitation published by Albert Einstein in May 1916 and is the accepted description of the gravitation of macroscopic objects in modern physics. General relativity generalizes special relativity and refines Isaac Newton's law of universal gravitation, providing a unified description of gravity as a geometric property of space and time, or four-dimensional spacetime. In particular, the curvature of spacetime is directly related to the energy, momentum, and stress of whatever is present, including matter and radiation. The relation is specified by the Einstein field equations, a system of second-order partial differential equations. John Archibald Wheeler summarized it: "Space-time tells matter how to move; matter tells space-time how to curve." Newton's law of universal gravitation, which describes gravity in classical mechanics, can be seen as a prediction of general relativity for the almost flat spacetime geometry around stationary mass distributions. Some predictions of general relativity, however, are beyond Newton's law of universal gravitation in classical physics. These predictions concern the passage of time, the geometry of space, the motion of bodies in free fall, and the propagation of light, and include gravitational time dilation, gravitational lensing, the gravitational redshift of light, the Shapiro time delay, singularities and black holes. So far, all tests of general relativity have been in agreement with the theory. The time-dependent solutions of general relativity enable us to extrapolate the history of the universe into the past and future, and have provided the modern framework for cosmology, thus leading to the discovery of the Big Bang and cosmic microwave background radiation. Despite the introduction of a number of alternative theories, general relativity continues to be the simplest theory consistent with experimental data. Reconciliation of general relativity with the laws of quantum physics remains a problem, however, as no self-consistent theory of quantum gravity has been found. It is not yet known how gravity can be unified with the three non-gravitational interactions: strong, weak and electromagnetic. Einstein's theory has astrophysical implications, including the prediction of black holes—regions of space in which space and time are distorted in such a way that nothing, not even light, can escape from them. Black holes are the end-state for massive stars. Microquasars and active galactic nuclei are believed to be stellar black holes and supermassive black holes. It also predicts gravitational lensing, where the bending of light results in distorted and multiple images of the same distant astronomical phenomenon. Other predictions include the existence of gravitational waves, which have been observed directly by the physics collaboration LIGO and other observatories. In addition, general relativity has provided the basis for cosmological models of an expanding universe. Widely acknowledged as a theory of extraordinary mathematical beauty, general relativity has often been described as the most beautiful of all existing physical theories.

Particle physics or high-energy physics is the study of fundamental particles and forces that constitute matter and radiation. The field also studies combinations of elementary particles up to the scale of protons and neutrons, while the study of combinations of protons and neutrons is called nuclear physics. The fundamental particles in the universe are classified in the Standard Model as fermions (matter particles) and bosons (force-carrying particles). There are three generations of fermions, although ordinary matter is made only from the first fermion generation. The first generation consists of up and down quarks which form protons and neutrons, and electrons and electron neutrinos. The three fundamental interactions known to be mediated by bosons are electromagnetism, the weak interaction, and the strong interaction. Quarks form hadrons, but cannot exist on their own. Hadrons that contain an odd number of quarks are called baryons and those that contain an even number are called mesons. Two baryons, the proton and the neutron, make up most of the mass of ordinary matter. Mesons are unstable and the longest-lived last for only a few hundredths of a microsecond. They occur after collisions between particles made of quarks, such as fast-moving protons and neutrons in cosmic rays. Mesons are also produced in cyclotrons or other particle accelerators. Particles have corresponding antiparticles with the same mass but with opposite electric charges. For example, the antiparticle of the electron is the positron. The electron has a negative electric charge, the positron has a positive charge. These antiparticles can theoretically form a corresponding form of matter called antimatter. Some particles, such as the photon, are their own antiparticle. These elementary particles are excitations of the quantum fields that also govern their interactions. The dominant theory explaining these fundamental particles and fields, along with their dynamics, is called the Standard Model. The reconciliation of gravity to the current particle physics theory is not solved; many theories have addressed this problem, such as loop quantum gravity, string theory and supersymmetry theory. Experimental particle physics is the study of these particles in radioactive processes and in particle accelerators such as the Large Hadron Collider. Theoretical particle physics is the study of these particles in the context of cosmology and quantum theory. The two are closely interrelated: the Higgs boson was postulated theoretically before being confirmed by experiments.

Plasma is a state of matter that results from one of the other three states (often, the gaseous one) having undergone an appreciable degree of ionization. It thus consists of a significant portion of charged particles (ions and/or electrons). While rarely encountered on Earth, it is estimated that 99.9% of all ordinary matter in the universe is plasma. Stars are almost pure balls of plasma, and plasma dominates the rarefied intracluster medium and intergalactic medium. Plasma can be artificially generated, for example, by heating a neutral gas or subjecting it to a strong electromagnetic field. The presence of charged particles makes plasma electrically conductive, with the dynamics of individual particles and macroscopic plasma motion governed by collective electromagnetic fields and very sensitive to externally applied fields. The response of plasma to electromagnetic fields is used in many modern devices and technologies, such as plasma televisions or plasma etching. Depending on the physical conditions, a certain number of neutral particles may also be present, in which case plasma is called partially ionized. Neon signs and lightning are examples of partially ionized plasmas. Unlike the phase transitions between the three other classical states of matter, the transition to plasma is not well defined and is a matter of interpretation and context. Whether a given degree of ionization suffices to call a substance "plasma" depends on the specific phenomenon being considered.

In physical cosmology and astronomy, dark energy is a proposed form of energy that affects the universe on its largest scales. Its primary effect is to drive the accelerating expansion of the universe. It also slows the rate of structure formation. Assuming that the lambda-CDM model of cosmology is correct, dark energy dominates the universe, contributing 68% of the total mass-energy in the present-day observable universe while dark matter and ordinary (baryonic) matter contribute 27% and 5%, respectively, and other components such as neutrinos and photons are nearly negligible. Dark energy's density is very low: 7×10−30 g/cm3 (6×10−10 J/m3 in mass-energy), much lower than the density of ordinary matter or dark matter within galaxies. However, it dominates the universe's mass–energy content because it is uniform across space. The first observational evidence for dark energy's existence came from measurements of supernovae. Type Ia supernovae have constant luminosity, which means they can be used to accurately measure distances. Comparing this distance to the redshift (which measures the speed at which the supernova is receding) shows that the universe's expansion is accelerating. Prior to this observation, scientists thought that the gravitational attraction of matter and energy in the universe would cause the universe's expansion to slow over time. Since the discovery of accelerating expansion, several independent lines of evidence have been discovered that support the existence of dark energy. The exact nature of dark energy remains a mystery, and many possible explanations have been theorized. The main candidates are a cosmological constant (representing a constant energy density filling space homogeneously) and scalar fields (dynamic quantities having energy densities that vary in time and space) such as quintessence or moduli. A cosmological constant would remain constant across time and space, while scalar fields can vary. Yet other possibilities are interacting dark energy (see the section Dark energy § Theories of dark energy), an observational effect, cosmological coupling, and shockwave cosmology (see the section § Alternatives to dark energy).

A black hole is an astronomical body so compact that its gravity prevents anything, including light, from escaping. Albert Einstein's theory of general relativity, which describes gravitation as the curvature of spacetime, predicts that any sufficiently compact mass will form a black hole. The boundary of no escape is called the event horizon. In general relativity, crossing a black hole's event horizon traps an object inside but produces no locally detectable change. General relativity also predicts that every black hole should have a central singularity, where the curvature of spacetime is infinite. Objects whose gravitational fields are too strong for light to escape were first considered in the 18th century. In 1916, the first solution of general relativity that would characterise a black hole was found. By the late 1950s, this solution began to be interpreted physically as a region of space from which nothing can escape. Black holes were long considered a mathematical curiosity; it was not until the 1960s that theoretical work showed they were a generic prediction of general relativity. The first widely accepted black hole was Cygnus X-1, identified by several researchers independently in 1971. Black holes typically form when very massive stars collapse at the end of their life cycle. After a black hole has formed, it can grow by absorbing mass from its surroundings. Supermassive black holes of millions of solar masses may form by absorbing stars and merging with other black holes, or via direct collapse of gas clouds. There is consensus that supermassive black holes exist in the centres of most galaxies. Quantum field theory in curved spacetime predicts that event horizons emit Hawking radiation, with its rate of emission being inversely proportional to its mass. This causes the black hole to lose mass very slowly, provided it is not accreting matter. However, even the smallest class of black holes observed, stellar black holes, are gaining mass from the cosmic microwave background faster than they are losing mass via Hawking radiation. The presence of a black hole can be inferred through its interaction with matter and electromagnetic radiation such as visible light. Matter falling toward a black hole can form an accretion disk of infalling plasma, heated by friction and emitting light. In extreme cases, this creates a quasar, some of the brightest objects in the universe. Merging black holes can be detected by the gravitational waves they emit. If stars are orbiting a black hole, their motions can be used to determine the black hole's mass and location. In this way, astronomers have identified numerous stellar black hole candidates in binary systems and established that the radio source known as Sagittarius A*, at the core of the Milky Way galaxy, contains a supermassive black hole of about 4.3 million solar masses.

The speed of light in vacuum, often called simply the speed of light and commonly denoted c, is a universal physical constant exactly equal to 299792458 m⋅s−1. It is exact because, by international agreement, a metre is defined as the length of the path travelled by light in vacuum during a time interval of 1⁄299792458 second. The value 299,792,458 metres per second is approximately 1 billion kilometres per hour; 700 million miles per hour. For other approximations of c valid for various units and size scales see the infobox. All forms of electromagnetic radiation, including visible light, travel in vacuum at the speed c as do massless particles and field perturbations, such as gravitational waves. The speed of light is the same for all observers, no matter their relative velocity. As a result, massless particles and waves travel at c in a vacuum regardless of the motion of the source or the inertial reference frame of the observer. The speed of light is the upper limit for the speed at which information, matter, or energy can travel through space. Particles with nonzero rest mass can be accelerated to approach c but can never reach it, regardless of the frame of reference in which their speed is measured. For long distances and sensitive measurements, the finite speed of light has noticeable effects. Much starlight viewed on Earth is from the distant past, allowing humans to study the history of the universe by viewing distant objects. When communicating with distant space probes, it can take hours for signals to travel. In computing, the speed of light fixes the ultimate minimum communication delay. The speed of light can be used in time of flight measurements to measure large distances to extremely high precision. Ole Rømer first demonstrated that light does not travel instantaneously by studying the apparent motion of Jupiter's moon Io. In an 1865 paper, James Clerk Maxwell proposed that light was an electromagnetic wave and, therefore, travelled at speed c. Albert Einstein postulated that the speed of light c with respect to any inertial frame of reference is a constant and is independent of the motion of the light source. He explored the consequences of that postulate by deriving the theory of relativity, and so showed that the parameter c had relevance outside of the context of light and electromagnetism. In the theory of relativity, c interrelates space and time and appears in the famous mass–energy equivalence, E = mc2. In some cases, objects or waves may appear to travel faster than light. The expansion of the universe is understood to exceed the speed of light beyond a certain boundary. The speed at which light propagates through transparent materials, such as glass or air, is less than c; similarly, the speed of electromagnetic waves around wire cables (the speed of electricity) is slower than c. The ratio between c and the speed v at which light travels in a material is called the refractive index n of the material (n = ⁠c/v⁠). For example, for visible light, the refractive index of glass is typically around 1.5, meaning that light in glass travels at ⁠c/1.5⁠ ≈ 200000 km/s (124000 mi/s); the refractive index of air for visible light is about 1.0003, so the speed of light in air is about 90 km/s (56 mi/s) slower than c.

In physics, the special theory of relativity, or simply special relativity, is a scientific theory of the relationship between space and time. In Albert Einstein's 1905 paper, "On the Electrodynamics of Moving Bodies", the theory is presented as being based on just two postulates: The laws of physics are invariant (identical) in all inertial frames of reference (that is, frames of reference with no acceleration). This is known as the principle of relativity. The speed of light in vacuum is the same for all observers, regardless of the motion of light source or observer. This is known as the principle of light constancy, or the principle of light speed invariance. The first postulate was first formulated by Galileo Galilei (see Galilean invariance).

The Big Bang is a physical theory that describes how the universe expanded from an initial state of high density and temperature. Various cosmological models based on the Big Bang concept explain a broad range of phenomena, including the abundance of light elements, the cosmic microwave background (CMB) radiation, the redshift of galaxies and the large-scale structure of the universe. The uniformity of the universe, known as the horizon and flatness problems, is explained through cosmic inflation: a phase of accelerated expansion during the earliest stages. Detailed measurements of the expansion rate of the universe place the initial singularity at an estimated 13.787±0.02 billion years ago, which is considered the age of the universe. A wide range of empirical evidence strongly favors the Big Bang event, which is now widely accepted. Extrapolating this cosmic expansion backward in time using the known laws of physics, the models describe an extraordinarily hot and dense primordial universe. Physics lacks a widely accepted theory that can model the earliest conditions of the Big Bang. As the universe expanded, it cooled sufficiently to allow the formation of subatomic particles, and later atoms. These primordial elements—mostly hydrogen, with some helium and lithium—then coalesced under the force of gravity aided by dark matter, forming early stars and galaxies. Measurements of the redshifts of supernovae indicate that the expansion of the universe is accelerating, an observation attributed to a concept called dark energy. The concept of an expanding universe was introduced by the physicist Alexander Friedmann in 1922 with the mathematical derivation of the Friedmann equations. The earliest empirical observation of an expanding universe is known as Hubble's law, published in work by physicist Edwin Hubble in 1929, which discerned that galaxies are moving away from Earth at a rate that accelerates proportionally with distance. Independent of Friedmann's work, and independent of Hubble's observations, in 1931 physicist Georges Lemaître proposed that the universe emerged from a "primeval atom", introducing the modern notion of the Big Bang. In 1964, the CMB was discovered. Over the next few years measurements showed this radiation to be uniform over directions in the sky and the shape of the energy versus intensity curve, both consistent with the Big Bang models of high temperatures and densities in the distant past. By the late 1960s most cosmologists were convinced that competing steady-state model of cosmic evolution was incorrect. There remain aspects of the observed universe that are not yet adequately explained by the Big Bang models. These include the unequal abundances of matter and antimatter known as baryon asymmetry, the detailed nature of dark matter surrounding galaxies, and the origin of dark energy.

Quantum mechanics is the fundamental physical theory that describes the behavior of matter and of light; its unusual characteristics typically occur at and below the scale of atoms. It is the foundation of all quantum physics, which includes quantum chemistry, quantum biology, quantum field theory, quantum technology, and quantum information science. Quantum mechanics can describe many systems that classical physics cannot. Classical physics can describe many aspects of nature at an ordinary (macroscopic and (optical) microscopic) scale, however is insufficient for describing them at very small submicroscopic (atomic and subatomic) scales. Classical mechanics can be derived from quantum mechanics as an approximation that is valid at ordinary scales. Quantum systems have bound states that are quantized to discrete values of energy, momentum, angular momentum, and other quantities, in contrast to classical systems where these quantities can be measured continuously. Measurements of quantum systems show characteristics of both particles and waves (wave–particle duality), and there are limits to how accurately the value of a physical quantity can be predicted prior to its measurement, given a complete set of initial conditions (the uncertainty principle). Quantum mechanics arose gradually from theories to explain observations that could not be reconciled with classical physics, such as Max Planck's solution in 1900 to the black-body radiation problem, and the correspondence between energy and frequency in Albert Einstein's 1905 paper, which explained the photoelectric effect. These early attempts to understand microscopic phenomena, now known as the "old quantum theory", led to the full development of quantum mechanics in the mid-1920s by Niels Bohr, Erwin Schrödinger, Werner Heisenberg, Max Born, Paul Dirac and others. The modern theory is formulated in various specially developed mathematical formalisms. In one of them, a mathematical entity called the wave function provides information, in the form of probability amplitudes, about what measurements of a particle's energy, momentum, and other physical properties may yield.

Entropy is a thermodynamic state variable that quantifies the probabilistic distribution of accessible microstates in a system. The term and the concept are used in diverse fields, from classical thermodynamics (where it was first recognized), to the microscopic description of nature in statistical physics, and the principles of information theory. It has found far-ranging applications in chemistry and physics, in biological systems and their relation to life, in cosmology, economics, and information systems including the transmission of information in telecommunication. Entropy is central to the second law of thermodynamics, which states that the entropy of an isolated system left to spontaneous evolution cannot decrease with time. As a result, isolated systems evolve toward thermodynamic equilibrium, where the entropy is highest. "High" entropy means that energy is more disordered or dispersed, while "low" entropy means that energy is more ordered or concentrated. A consequence of the second law of thermodynamics is that certain processes are irreversible. The thermodynamic concept was referred to by Scottish scientist and engineer William Rankine in 1850 with the names thermodynamic function and heat-potential. In 1865, German physicist Rudolf Clausius, one of the leading founders of the field of thermodynamics, defined it as the quotient of an infinitesimal amount of heat to the instantaneous temperature. He initially described it as transformation-content, in German Verwandlungsinhalt, and later coined the term entropy from a Greek word for transformation. Austrian physicist Ludwig Boltzmann explained entropy as the measure of the number of possible microscopic arrangements or states of individual atoms and molecules of a system that comply with the macroscopic condition of the system. He thereby introduced the concept of statistical disorder and probability distributions into a new field of thermodynamics, called statistical mechanics, and found the link between the microscopic interactions, which fluctuate about an average configuration, to the macroscopically observable behaviour, in the form of a simple logarithmic law, with a proportionality constant, the Boltzmann constant, which has become one of the defining universal constants for the modern International System of Units.

Nuclear fusion is a reaction in which two or more atomic nuclei combine to form a larger nucleus. The difference in mass between the reactants and products is manifested as either the release or the absorption of energy. This difference in mass arises as a result of the difference in nuclear binding energy between the atomic nuclei before and after the fusion reaction. Nuclear fusion is the process that powers all active stars, via many reaction pathways. Fusion processes require an extremely large triple product of temperature, density, and confinement time. These conditions occur only in stellar cores, advanced nuclear weapons, and are approached in fusion power experiments. A nuclear fusion process that produces atomic nuclei lighter than nickel-62 is generally exothermic, due to the positive gradient of the nuclear binding energy curve. The most fusible nuclei are among the lightest, especially deuterium, tritium, and helium-3. The opposite process, nuclear fission, is most energetic for very heavy nuclei, especially the actinides. Applications of fusion include fusion power, thermonuclear weapons, boosted fission weapons, neutron sources, and superheavy element production.

A neutron star is the gravitationally collapsed core of a massive supergiant star. It results from the supernova explosion of a massive star—combined with gravitational collapse—that compresses the core past white dwarf star density to that of atomic nuclei. Surpassed only by black holes, neutron stars are the second-smallest- and second-densest-known class of stellar objects. Neutron stars have a radius on the order of 10 kilometers (6 miles) and a mass of about 1.4 solar masses (M☉). Stars that collapse into neutron stars typically have an initial total mass between 10 and 25 M☉ or possibly more for those that are especially rich in elements heavier than hydrogen and helium. There are thought to be around one billion neutron stars in the Milky Way, and at a minimum several hundred million, a figure obtained by estimating the number of stars that have undergone supernova explosions. However, many of them have existed for a long period of time and have cooled down considerably. Originally it was thought that neutron stars would be difficult to detect due to low emissions. However, it was discovered that spinning stars emit radiation. Most neutron stars that have been detected are pulsars or a part of a binary system. Neutron stars in a binary system with a main sequence star can pull in large amounts of gas from its companion, a process called accretion. These binary systems continue to evolve, with many companions eventually becoming compact objects such as white dwarfs or neutron stars themselves, though other possibilities include a complete destruction of the companion through ablation or collision. The study of neutron star systems is central to gravitational wave astronomy. The merger of binary neutron stars produces gravitational waves and is associated with kilonovae and short gamma-ray bursts. In 2017, the LIGO and Virgo interferometer sites observed GW170817, the first direct detection of gravitational waves from such an event. Prior to this, indirect evidence for gravitational waves was inferred by studying the gravity radiated from the orbital decay of a different type of (unmerged) binary neutron system, the Hulse–Taylor pulsar.

In physics, electromagnetic radiation (EMR) or an electromagnetic wave (EMW) is a self-propagating wave of the electromagnetic field that carries momentum and radiant energy through space. It encompasses a broad spectrum, classified by frequency (inversely proportional to wavelength), ranging from radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, to gamma rays. All forms of EMR travel at the speed of light in a vacuum and exhibit wave–particle duality, behaving both as waves and as discrete particles called photons. Electromagnetic radiation is produced by accelerating charged particles such as from the Sun and other celestial bodies or artificially generated for various applications. Its interaction with matter depends on wavelength, influencing its uses in communication, medicine, industry, and scientific research. Radio waves enable broadcasting and wireless communication, infrared is used in thermal imaging, visible light is essential for vision, and higher-energy radiation, such as X-rays and gamma rays, is applied in medical imaging, cancer treatment, and industrial inspection. Higher energy EMR, especially in the UV and above, has been associated with negative health effects. In quantum mechanics, an alternate way of viewing EMR is that it consists of photons, uncharged elementary particles with zero rest mass which are the quanta of the electromagnetic field, responsible for all electromagnetic interactions. Quantum electrodynamics is the theory of how EMR interacts with matter on an atomic level. Quantum effects provide additional sources of EMR, such as the transition of electrons to lower energy levels in an atom and black-body radiation.

In physics, gravity (from Latin gravitas 'weight'), also known as gravitation or a gravitational interaction, is a fundamental interaction, which may be described as the force that draws material objects towards each other. The gravitational attraction between clouds of primordial hydrogen and clumps of dark matter in the early universe caused the hydrogen gas to coalesce, eventually condensing and fusing to form stars. At larger scales this resulted in galaxies and clusters, so gravity is a primary driver for the large-scale structures in the universe. Gravity has an infinite range, although its effects become weaker as objects get farther away. Gravity is described by the general theory of relativity, proposed by Albert Einstein in 1915, which describes gravity in terms of the curvature of spacetime, caused by the uneven distribution of mass. The most extreme example of this curvature of spacetime is a black hole, from which nothing—not even light—can escape once past the black hole's event horizon. However, for most applications, gravity is sufficiently well approximated by Newton's law of universal gravitation, which describes gravity as an attractive force between any two bodies that is proportional to the product of their masses and inversely proportional to the square of the distance between them. Scientists are looking for a theory that describes gravity in the framework of quantum mechanics (quantum gravity), which would unify gravity and the other known fundamental interactions of physics in a single mathematical framework (a theory of everything). On the surface of a planetary body such as on Earth, the force of gravity operates towards the center of the body and is modified by the centrifugal effects arising from the rotation of the body. In this context, gravity gives weight to physical objects and is essential to understanding the mechanisms that are responsible for surface water waves, lunar tides and substantially contributes to weather patterns. Gravitational weight also has many important biological functions, helping to guide the growth of plants through the process of gravitropism and influencing the circulation of fluids in multicellular organisms.

The Higgs boson, sometimes called the Higgs particle, is an elementary particle in the Standard Model of particle physics produced by the quantum excitation of the Higgs field, one of the fields in particle physics theory. In the Standard Model, the Higgs particle is a massive scalar boson that couples to (interacts with) particles whose mass arises from their interactions with the Higgs field, has zero spin, even (positive) parity, no electric charge, and no color charge. It is also very unstable, decaying into other particles almost immediately upon generation. The Higgs field is a scalar field with two neutral and two electrically charged components that form a complex doublet of the weak isospin SU(2) symmetry. Its "sombrero potential" leads it to take a nonzero value everywhere (including otherwise empty space), which breaks the weak isospin symmetry of the electroweak interaction and, via the Higgs mechanism, gives a rest mass to all massive elementary particles of the Standard Model, including the Higgs boson itself. The existence of the Higgs field became the last unverified part of the Standard Model of particle physics, and for several decades was considered "the central problem in particle physics". Both the field and the boson are named after physicist Peter Higgs, who in 1964, along with five other scientists in three teams, proposed the Higgs mechanism, a way for some particles to acquire mass. All fundamental particles known at the time should be massless at very high energies, but fully explaining how some particles gain mass at lower energies had been extremely difficult. If these ideas were correct, a particle known as a scalar boson (with certain properties) should also exist. This particle was called the Higgs boson and could be used to test whether the Higgs field was the correct explanation. After a 40-year search, a subatomic particle with the expected properties was discovered in 2012 by the ATLAS and CMS experiments at the Large Hadron Collider (LHC) at CERN near Geneva, Switzerland. The new particle was subsequently confirmed to match the expected properties of a Higgs boson. Physicists from two of the three teams, Peter Higgs and François Englert, were awarded the Nobel Prize in Physics in 2013 for their theoretical predictions. Although Higgs's name has come to be associated with this theory, several researchers between about 1960 and 1972 independently developed different parts of it. In the media, the Higgs boson has often been called the "God particle" after the 1993 book The God Particle by Nobel Laureate Leon M. Lederman. The name has been criticised by physicists, including Peter Higgs.

Liberalism is a political and moral philosophy based on the rights of the individual, liberty, consent of the governed, political equality, right to private property, and equality before the law. Different liberals espouse various and sometimes conflicting views depending on their understanding of these principles but generally support liberal democracy, private property, market economies, individual rights (including civil rights and human rights), secularism, rule of law, economic and political freedom, freedom of speech, freedom of the press, freedom of assembly, and freedom of religion. Liberalism is frequently cited as the dominant ideology of modern history. Liberalism became a distinct movement in the Age of Enlightenment, gaining popularity among Western philosophers and economists. Liberalism sought to replace the norms of hereditary privilege, state religion, absolute monarchy, the divine right of kings, and traditional conservatism with representative democracy, rule of law, and equality under the law. Liberals also ended mercantilist policies, royal monopolies, and other trade barriers, instead promoting free trade and marketization. The philosopher John Locke is often credited with founding liberalism as a distinct tradition based on the social contract, arguing that each man has a natural right to life, liberty and property, and governments must not violate these rights. While the British liberal tradition emphasized expanding democracy, French liberalism emphasized rejecting authoritarianism. Leaders in the British Glorious Revolution of 1688, the American Revolution of 1776, and the French Revolution of 1789 used liberal philosophy to justify the armed overthrow of royal sovereignty. The 19th century saw liberal governments established in Europe and South America, and it was well-established alongside republicanism in the United States. In Victorian Britain, it was used to critique the political establishment, appealing to science and reason on behalf of the people. During the 19th and early 20th centuries, liberalism in the Ottoman Empire and the Middle East influenced periods of reform, such as the Tanzimat and Al-Nahda, and the rise of constitutionalism, nationalism, and secularism. These changes, along with other factors, helped to create a sense of crisis within Islam, which continues to this day, leading to Islamic revivalism. Before 1920, the main ideological opponents of liberalism were communism, conservatism, and socialism; liberalism then faced major ideological challenges from fascism and Marxism–Leninism. During the 20th century, liberal ideas spread further, especially in Western Europe, as liberal democracies won both world wars and the Cold War. Liberals sought and established a constitutional order that prized important individual freedoms, such as freedom of speech and freedom of association; an independent judiciary and public trial by jury; and the abolition of aristocratic privileges. Later waves of modern liberal thought and struggle were strongly influenced by the need to expand civil rights. Liberals have advocated gender and racial equality in their drive to promote civil rights, and global civil rights movements in the 20th century achieved several objectives towards both goals. Other goals often accepted by liberals include universal suffrage and universal access to education. In Europe and North America, the establishment of social liberalism (often called simply liberalism in the United States) became a key component in expanding the welfare state. 21st-century liberal parties continue to wield power and influence throughout the world. The fundamental elements of contemporary society have liberal roots. The early waves of liberalism popularised economic individualism while expanding constitutional government and parliamentary authority.

Oligarchy (from Ancient Greek ὀλιγαρχία (oligarkhía) 'rule by few'; from ὀλίγος (olígos) 'few' and ἄρχω (árkhō) 'to rule, command') is a form of government in which power rests with a small number of people. Leaders of such regimes are often referred to as oligarchs, and generally are characterized by having titles of nobility or high amounts of wealth.

Communism (from Latin communis 'common, universal') is a political and economic ideology whose goal is the creation of a communist society, a socioeconomic order centered on common ownership of the means of production, distribution, and exchange that allocates products in society based on need. A communist society entails the absence of private property and social classes, and ultimately money and the state. Communism is a part of the broader socialist movement. Communists often seek a voluntary state of self-governance but disagree on the means to this end. This reflects a distinction between a libertarian socialist approach of communization, revolutionary spontaneity, and workers' self-management, and an authoritarian socialist, vanguardist, or party-driven approach to establish a socialist state, which is expected to wither away. There are many variants of communism, such as anarchist communism, Marxist schools of thought (including Leninism and its offshoots), and religious communism. These ideologies share the analysis that the current order of society stems from the capitalist economic system and mode of production; they believe that there are two major social classes under capitalism, that the relationship between them is exploitative, and that it can only be resolved through social revolution. The two classes under capitalism are the proletariat (working class), who make up most of the population and sell their labor power to survive, and the bourgeoisie (owning class), a minority that derives profit from employing the proletariat through private ownership of the means of production. Some variants additionally emphasize feudal classes, such as the peasantry and feudal lords, or other classes. According to this, a communist revolution would put the working class in power, and establish common ownership of property, the primary element in the transformation of society towards a socialist mode of production. Communism in its modern form grew out of the socialist movement in 19th-century Europe that argued capitalism caused the misery of urban factory workers. In 1848, Karl Marx and Friedrich Engels offered a new definition of communism in The Communist Manifesto. In the 20th century, Communist governments espousing Marxism–Leninism came to power, first in the Soviet Union with the 1917 Russian Revolution, then in Eastern Europe, Asia, and other regions after World War II. By the 1920s, communism had become one of the two dominant types of socialism in the world, the other being social democracy. For much of the 20th century, more than one-third of the world's population lived under Communist governments. These were characterized by one-party rule, rejection of private property and capitalism, state control of economic activity and mass media, restrictions on freedom of religion, and suppression of opposition. With the dissolution of the Soviet Union in 1991, many governments abolished Communist rule. Only a few nominally Communist governments remain, such as China, Cuba, Laos, North Korea, and Vietnam. Except for North Korea, these have allowed more economic competition while maintaining one-party rule. Communism's decline has been attributed to economic inefficiency and to authoritarianism and bureaucracy within Communist governments. Although the emergence of the Soviet Union as the first state based on Marxism–Leninism led to socialism becoming widely associated with the Soviet economic model, several scholars argue that in practice this model functioned as a form of state capitalism. Public memory of 20th-century Marxist-Leninist states has been described as a battleground between anti-communism and anti anti-communism. Authors have written about mass killings under communist regimes and mortality rates, which remain controversial, polarized, and debated topics in academia, historiography, and politics when discussing communism and the legacy of Marxist-Leninist states. From the 1990s, many Communist parties adopted democratic principles and came to share power with others in government, such as the CPN UML and the Nepal Communist Party, which support People's Multiparty Democracy in Nepal.

An election is a formal group decision-making process whereby a portion or all of a population or group votes to choose an individual or multiple individuals to hold public office or other position of responsibility. Elections have been the usual mechanism by which modern representative democracy has operated since the 17th century. Elections may fill offices in the legislature, sometimes in the executive and judiciary, and for regional and local government, such as cities or towns. This process is also used in many other Standardized Associations, public businesses, and organizations, from clubs to voluntary association and corporations. The global use of elections as a tool for selecting representatives in modern representative democracies is in contrast with the practice in the democratic archetype, ancient Athens, where the elections were considered an oligarchic institution and most political offices were filled using allotment which is also known as "Sortition", by which office holders were chosen by lot. Electoral reform describes the process of introducing fair electoral systems where they are not in place, or improving the fairness or effectiveness of existing systems. Psephology is the study of results and other statistics relating to elections (especially with a view to predicting future results). The term elect means "to select or to nominate", so sometimes other forms of balloting such as referendums are referred to as elections, especially in the United States.

Human rights are universally recognized moral principles or norms that establish standards of human behavior and are often protected by both national and international laws. These rights are considered inherent and inalienable, meaning they belong to every individual simply by virtue of being human. They encompass a broad range of civil, political, economic, social, and cultural rights, such as the right to life, freedom of speech, protection against enslavement, and right to education. While ideas related to human rights predate modernity, the modern concept of human rights gained significant prominence after World War II, particularly in response to the atrocities of the Holocaust, leading to the adoption of the Universal Declaration of Human Rights (UDHR) by the United Nations General Assembly in 1948. This document outlined a comprehensive framework of rights that countries are encouraged to protect, setting a global standard for human dignity, freedom, and justice. The Universal Declaration of Human Rights (UDHR) has since inspired numerous international treaties and national laws aimed at promoting and protecting human rights worldwide. While the principle of universal human rights is widely accepted, debates persist regarding which rights should take precedence, how they should be implemented, and their applicability in different cultural contexts. Criticisms often arise from perspectives like cultural relativism, which argue that individual human rights are inappropriate for societies that prioritize a communal or collectivist identity, and may conflict with certain cultural or traditional practices. Nonetheless, human rights remain a central focus in international relations and legal frameworks, supported by institutions such as the United Nations, various non-governmental organizations, and national bodies dedicated to monitoring and enforcing human rights standards worldwide.

Imperialism is the maintaining and extending of power over foreign nations, particularly through expansionism, employing both hard power (military and economic power) and soft power (diplomatic power and cultural imperialism). Imperialism focuses on establishing or maintaining hegemony and a more formal empire. While related to the concept of colonialism, imperialism is a distinct concept that can apply to other forms of expansion and many forms of government. Kenneth Waltz suggests that the root cause of imperialism is great power. He explains that "[w]here one finds empires, one notices that they are built by those who have organized themselves and exploited their resources most effectively," adding that "[w]eakness invites control; strength tempts one to exercise it, even if only for the 'good' of other people." According to Vladimir Lenin, imperialist wars are inevitable due to uneven development under capitalism.

Nationalism is an ideology or movement that holds that the nation should be congruent with the state. As a movement, it presupposes the existence and tends to promote the interests of a particular nation, especially with the aim of gaining and maintaining its sovereignty (self-determination) over its perceived homeland to create a nation-state. It holds that the nation should govern itself, free from outside interference (self-governance), that a nation is a natural and ideal basis for a polity, and that the nation is the only rightful source of political power. It further aims to build, and maintain, a single national identity, based on a combination of shared social characteristics such as culture, ethnicity, homeland, language, politics (or government), religion, traditions, or belief in a shared singular history, and to promote national unity or solidarity. There are various definitions of a "nation", which leads to different types of nationalism. The two main divergent forms are ethnic nationalism and civic nationalism. The moral value of nationalism, the relationship between nationalism and patriotism, and the compatibility of nationalism and cosmopolitanism are all subjects of philosophical debate. Nationalism can be combined with diverse political goals and ideologies, such as conservatism (national conservatism and right-wing populism) or socialism (left-wing nationalism). Nationalism is seen as positive or negative depending on its ideology and outcomes. Nationalism has been a feature of movements for freedom and justice, has been associated with cultural revivals, and encourages pride in national achievements. It has also been used to legitimize racial, ethnic, and religious divisions, suppress or attack minorities, undermine human rights and democratic traditions, and start wars, being frequently cited as a cause of both world wars.

The European Union (EU) is a political and economic union of 27 member states that are located primarily in Europe. A supranational union with a total area of 4,233,255 km2 (1,634,469 sq mi) and an estimated population of over 450 million as of 2025, its member states generated a nominal gross domestic product (GDP) of around €18.802 trillion in 2025, accounting for approximately one sixth of global economic output. The EU is often described as a sui generis political entity combining characteristics of both a federation and a confederation. Its cornerstone, the Customs Union, paved the way to establishing an internal single market based on standardised legal framework and legislation that applies in all member states in those matters, and only those matters, where the states have agreed to act as one. EU policies aim to ensure the free movement of people, goods, services and capital within the internal market; enact legislation in justice and home affairs; and maintain common policies on trade, agriculture, fisheries, and regional development. Passport controls have been abolished for travel within the Schengen Area. The eurozone is a group composed of the 21 EU member states that have fully implemented the EU's economic and monetary union and use the euro currency. Through the Common Foreign and Security Policy, the union has developed a role in external relations and defence. It maintains permanent diplomatic missions throughout the world and represents itself at the United Nations, the World Trade Organization, the G7 and the G20. In 2012, the EU was awarded the Nobel Peace Prize. The EU was established, along with its citizenship, when the Maastricht Treaty came into force in 1993, and was incorporated as an international legal juridical person upon entry into force of the Treaty of Lisbon in 2009. Its beginnings can be traced to the Inner Six states (Belgium, France, Italy, Luxembourg, the Netherlands, and West Germany) at the start of modern European integration in 1948, and to the Western Union, the International Authority for the Ruhr, the European Coal and Steel Community, the European Economic Community and the European Atomic Energy Community, which were established by treaties. These increasingly amalgamated bodies grew, with their legal successor the EU, both in size through the accessions of a further 22 states from 1973 to 2013, and in power through acquisitions of policy areas. In 2020, the United Kingdom became the only member state to leave the EU; ten countries are aspiring or negotiating to join it.

A prime minister, also known as a chief of cabinet, chief minister, first minister, minister-president or premier, is the head of the cabinet and the leader of the ministers in the executive branch of government, often in a parliamentary or semi-presidential system. A prime minister is not the head of state, but rather the head of government, serving as the chief of the executive under either a monarch or a president in a republican form of government. In parliamentary systems of government (be they constitutional monarchies or parliamentary republics), the Prime Minister (or occasionally a similar post with a different title, such as the Chancellor of Germany) is the most powerful politician and the functional leader of the state, by virtue of commanding the confidence of the legislature. The head of state is typically a ceremonial officer, though they may exercise reserve powers to check the Prime Minister in unusual situations. Under some presidential systems, such as South Korea and Peru, the prime minister is the leader or the most senior member of the cabinet, but not the head of government. As such, in South Korea, the prime minister is equivalent to that of a vice president which is the second in-command and assumes the presidency in the absence of the president. In many systems, the prime minister selects and may dismiss other members of the cabinet, and allocates posts to members within the government. In most systems, the prime minister is the presiding member and chairman of the cabinet. In a minority of systems, notably in semi-presidential systems, a prime minister is the official appointed to manage the civil service and execute the directives of the head of state. Today, the prime minister is often, but not always, a member of the legislature or its lower house, and is expected with other ministers to ensure the passage of bills through the legislature. In some monarchies the monarch may also exercise executive powers (known as the royal prerogative) without the approval of parliament. As well as being head of government, being prime minister may require holding other roles or posts—the prime minister of the United Kingdom, for example, is also First Lord of the Treasury and Minister for the Civil Service. In some cases, prime ministers may choose to hold additional ministerial posts (e.g. when the portfolio is critical to that government's mandate): during the Second World War Winston Churchill was also Minister of Defence (although there was then no Ministry of Defence). Another example is the Thirty-fourth government of Israel (2015–2019), when Benjamin Netanyahu at one point served as the prime minister and minister of Communications, Foreign Affairs, Regional Cooperation, Economy, Defense and Interior.

Authoritarianism is a political system characterized by the rejection of political plurality, the use of strong central power to preserve the political status quo, and reductions in democracy, separation of powers, civil liberties, and the rule of law. Authoritarian regimes may be either autocratic or oligarchic and may be based upon the rule of a party, the military, or the concentration of power in a single person. States that have a blurred boundary between democracy and authoritarianism have sometimes been characterized as "hybrid democracies", "hybrid regimes" or "competitive authoritarian" states. The political scientist Juan Linz, in an influential 1964 work, An Authoritarian Regime: Spain, defined authoritarianism as possessing four qualities: Limited political pluralism, which is achieved with constraints on the legislature, political parties and interest groups. Political legitimacy based on appeals to emotion and identification of the regime as a necessary evil to combat "easily recognizable societal problems, such as underdevelopment or insurgency." Minimal political mobilization, and suppression of anti-regime activities. Ill-defined executive powers, often vague and shifting, used to extend the power of the executive. Minimally defined, an authoritarian government lacks free and competitive direct elections to legislatures, free and competitive direct or indirect elections for executives, or both. Broadly defined, authoritarian states include countries that lack human rights such as freedom of religion, or countries in which the government and the opposition do not alternate in power at least once following free elections. Authoritarian states might contain nominally democratic institutions such as political parties, legislatures and elections which are managed to entrench authoritarian rule and can feature fraudulent, non-competitive elections. Since 1946, the share of authoritarian states in the international political system increased until the mid-1970s but declined from then until 2000. Prior to 2000, dictatorships typically began with a coup and replaced a pre-existing authoritarian regime. Since 2000, dictatorships are most likely to begin through democratic backsliding whereby a democratically elected leader established an authoritarian regime.

A political party is an organization that coordinates candidates to compete in elections and participate in governance. It is common for the members of a party to hold similar ideas about politics, and parties may promote specific ideological or policy goals. Political parties have become a major part of the politics of almost every country, as modern party organizations developed and spread around the world over the last few centuries. Although some countries have no political parties, it is extremely rare. Most countries have several parties while others only have one. Parties are important in the politics of autocracies as well as democracies, though usually democracies have more political parties than autocracies. Autocracies often have a single party that governs the country, and some political scientists consider competition between two or more parties to be an essential part of democracy. Parties can develop from existing divisions in society, like the divisions between lower and upper classes, and they streamline the process of making political decisions by encouraging their members to cooperate. Political parties usually include a party leader, who has primary responsibility for the activities of the party; party executives, who may select the leader and who perform administrative and organizational tasks; and party members, who may volunteer to help the party, donate money to it, and vote for its candidates. There are many different ways in which political parties can be structured and interact with the electorate. The contributions that citizens give to political parties are often regulated by law, and parties will sometimes govern in a way that favours the people who donate time and money to them. In some cases, a political party tightly cooperates with affiliated or subordinate organizations of a different type, such as a trade union, a youth organization, or a party militia (armed wing). Many political parties are motivated by ideological goals. It is common for democratic elections to feature liberal, conservative, and socialist parties; other common ideologies of very large political parties include communism, populism, nationalism, and Islamism. Political parties in different countries will often adopt similar colours and symbols to identify themselves with a particular ideology. However, many political parties have no ideological affiliation and may instead be primarily engaged in patronage, clientelism, the advancement of a specific political entrepreneur, or be a "big tent", in that they wish to attract voters who have a variety of positions on issues.

Federalism is a mode of government that combines a general level of government (a central or federal government) with a regional level of sub-unit governments (e.g., counties, provinces, states, cantons, territories, etc.), while dividing the powers of governing between the two levels of governments. Johannes Althusius (1563–1638) is considered the father of modern federalism, along with Montesquieu. By 1748, in his treatise The Spirit of Law, Montesquieu (1689–1755) observed various examples of federalist governments: in corporate societies, in the polis bringing villages together, and in cities themselves forming confederations. In the modern era, federalism was first adopted by a union of the states of the Old Swiss Confederacy as of the mid-14th century. Federalism differs from confederalism, where the central government is created subordinate to the regional states—and is notable for its regional separation of governing powers (e.g., in the United States, the Articles of Confederation as the general level of government of the original Thirteen Colonies). Federalism also differs from the unitary state, where the regional level is subordinate to the central government, even after a devolution of powers (e.g., the United Kingdom). Examples of federalism today include: Argentina, Australia, Brazil, Canada, Ethiopia, Germany, India, Iraq, Malaysia, Mexico, Switzerland, the United Arab Emirates, and the United States. At a multi-state level, the European Union has been characterized by some as a federation.

The North Atlantic Treaty Organization (NATO) is an intergovernmental military alliance between 32 member states—30 in Europe and two in North America. Founded in the aftermath of World War II, NATO was established with the signing of the North Atlantic Treaty in 1949. The organization serves as a system of collective security and deterrence, whereby its independent members agree to defend each-other from attack by any outside party. This is enshrined in Article 5 of the treaty, which states that an armed attack against the territory of one member shall be considered an attack against them all. Throughout the Cold War, NATO's primary purpose was to deter and counter the threat posed by the Soviet Union and its satellite states, which formed the rival Warsaw Pact in 1955. Following the dissolution of the Soviet Union in 1991, the alliance adapted. NATO undertook its first military intervention in the Bosnian War (1992–1995); with a United Nations mandate, it enforced a no-fly zone and defended UN safe zones. NATO also intervened in the Kosovo War in 1999 by bombing Yugoslavia with the stated goal of ending the violence and repressive actions by the Milošević government. Article 5 was invoked for the first and only time by the United States, after the September 11 attacks, leading to the NATO invasion of Afghanistan in 2001, as part of the International Security Assistance Force (ISAF). In 2011, NATO intervened in the Libyan civil war, with a UN mandate to enforce a no-fly zone and protect civilians. The alliance has since been involved in training Iraq's military and countering piracy. Since the end of the Cold War, the alliance has accepted sixteen new members in Europe. NATO recognizes Bosnia and Herzegovina, Ukraine and Georgia as aspiring members. Russia–NATO relations had been co-operative, but became adversarial during Vladimir Putin's second presidency. Putin called NATO's eastward enlargement a threat, and opposes Ukraine joining the alliance. Following the Russian annexation of Crimea in 2014, NATO suspended co-operation and deployed a tripwire force in the east. The 2022 Russian invasion of Ukraine prompted NATO to reinforce its eastern flank and send military aid to Ukraine. The invasion caused Finland and Sweden to join the alliance. Since the invasion, Russia has been accused of waging a hybrid conflict against NATO. Since 2025, there has been internal crisis in NATO due to threats made by the United States, a major contributing member, against the sovereignty of fellow members Canada and Denmark. NATO members contribute to collective defense in two ways: by funding their own defense, and by helping to fund a shared budget for running the organization as a whole and maintaining joint facilities. Each member has committed to a target of spending at least 5% of their gross domestic product (GDP) on their own defense, while in 2026 the shared budget was around €5 billion split between all 32 members. As of 2026, the United States spends the most overall on its defense, Norway spends the most per capita, while Poland and the Baltic states spend the most as a share of GDP. The combined military spending of NATO members makes up over half of the world total. The combined militaries of all NATO members include about 3.5 million soldiers and personnel. NATO's main headquarters are in Brussels, Belgium, while its military headquarters are near Mons, Belgium.

Geopolitics (from Ancient Greek γῆ gê 'earth, land' and πολιτική politikḗ 'politics') is the study of the effects of Earth's geography on politics and international relations. Geopolitics usually refers to countries and relations between them. According to multiple researchers, the term is currently being used to describe a broad spectrum of concepts, in a general sense used as "a synonym for international political relations", but more specifically "to imply the global structure of such relations"; this usage builds on an "early-twentieth-century term for a pseudoscience of political geography" and other pseudoscientific theories of historical and geographic determinism. At the level of international relations, geopolitics is a method of studying foreign policy to understand, explain, and predict international political behavior through geographical variables. These include area studies, climate, topography, demography, natural resources, and applied science of the region being evaluated. Geopolitics focuses on political power linked to geographic space, in particular, territorial waters, land territory and wealth of natural resources, in correlation with diplomatic history, in particular the context of a larger power relative to its neighboring states of smaller or similar power. Some scholars have argued that geopolitics should serve as "an aid to statecraft." Topics of geopolitics include relations between the interests of international political actors focused within an area, a space, or a geographical element, relations that create a geopolitical system. Critical geopolitics deconstructs classical geopolitical theories, by showing their political or ideological functions for great powers. There are some works that discuss the geopolitics of renewable energy. The relationship between geopolitics and geoeconomics is often analyzed by two main schools of thought: the strategic school and the political-economic school. The Austro-Hungarian historian Emil Reich (1854–1910) is considered to be the first to have coined the term in English as early as 1902 and later published in England in 1904 in his book Foundations of Modern Europe.

Conservatism is a cultural, social, and political philosophy and ideology that seeks to promote and preserve traditional institutions, customs, and values. The central tenets of conservatism may vary in relation to the culture and civilization in which it appears. In Western culture, depending on the particular nation and the particular time period, conservatives seek to promote and preserve a range of institutions, such as the nuclear family, organized religion, the military, the nation-state, property rights, rule of law, aristocracy, and monarchy. The 18th-century Anglo-Irish statesman Edmund Burke, who opposed the French Revolution but supported the American Revolution, is credited as one of the forefathers of conservative thought in the 1790s along with Savoyard statesman Joseph de Maistre. The first established use of the term in a political context originated in 1818 with François-René de Chateaubriand during the period of Bourbon Restoration that sought to roll back the policies of the French Revolution and establish social order. Conservatism has varied considerably as it has adapted itself to existing traditions and national cultures. Thus, conservatives from different parts of the world, each upholding their respective traditions, may disagree on a wide range of issues. One of the three major ideologies along with liberalism and socialism, conservatism is the dominant ideology in many nations across the world, including Hungary, India, Iran, Israel, Italy, Japan, Poland, Russia, United States, Singapore, and South Korea. Historically associated with right-wing politics, the term has been used to describe a wide range of views. Conservatism may be either libertarian or authoritarian, populist or elitist, progressive or reactionary, moderate or extreme.

The president of the United States (POTUS) is the head of state and head of government of the United States. The president directs the executive branch of the federal government and is the commander-in-chief of the United States Armed Forces. The power of the presidency has grown since the first president, George Washington, took office in 1789. While presidential power has ebbed and flowed over time, the presidency has played an increasing role in American political life since the beginning of the 20th century, carrying over into the 21st century with some expansions during the presidencies of Franklin D. Roosevelt and George W. Bush. In the 21st century, the president is one of the world's most powerful political figures and the leader of a superpower. As the leader of the nation with the largest economy by nominal GDP, the president possesses significant domestic and international hard and soft power. For much of the 20th century, especially during the Cold War, the U.S. president was often called "the leader of the free world". Article II of the Constitution establishes the executive branch of the federal government and vests executive power in the president. The power includes the execution and enforcement of federal law and the responsibility to appoint federal executive, diplomatic, regulatory, and judicial officers. Based on constitutional provisions empowering the president to appoint and receive ambassadors and conclude treaties with foreign powers, and on subsequent laws enacted by Congress, the modern presidency has primary responsibility for conducting U.S. foreign policy. The role includes responsibility for directing the world's most expensive military, which has the second-largest nuclear arsenal. The president also plays a leading role in federal legislation and domestic policymaking. As part of the system of separation of powers, Article I, Section 7 of the Constitution gives the president the power to sign or veto federal legislation. Since modern presidents are typically viewed as leaders of their political parties, major policymaking is significantly shaped by the outcome of presidential elections, with presidents taking an active role in promoting their policy priorities to members of Congress who are often electorally dependent on the president. Over time, presidents have also made increasing use of executive orders, agency regulations, and judicial appointments to shape domestic policy. The president is elected through the Electoral College to a four-year term, along with the vice president. Under the Twenty-second Amendment, ratified in 1951, no person who has been elected to two presidential terms may be elected to a third. In addition, nine vice presidents have become president by virtue of a president's intra-term death or resignation. In all, 45 individuals have served, or are serving, 47 presidencies as of the current 60th four-year term. Donald Trump is the 47th and current president since January 20, 2025.

Fascism ( FASH-iz-əm) is a far-right, authoritarian, and ultranationalist political ideology and movement that rose to prominence in early-20th-century Europe. Fascism is characterized by support for a dictatorial leader, centralized autocracy, militarism, forcible suppression of opposition, belief in a natural social hierarchy, subordination of individual interests for the perceived interest of the nation or race, and strong regimentation of society and the economy. Opposed to communism, democracy, liberalism, pluralism, and socialism, fascism is at the far-right of the traditional left–right spectrum. What constitutes a precise definition of fascism has been a longrunning and complex debate among scholars. The first fascist movements emerged in Italy during World War I before spreading to other European countries, most notably Germany. Fascism also had adherents outside of Europe. Fascists saw World War I as a revolution that brought massive changes to the nature of war, society, the state, and technology. The advent of total war and the mass mobilization of society erased the distinction between civilians and combatants. A military citizenship arose, in which all citizens were involved with the military in some manner. The war resulted in the rise of a powerful state capable of mobilizing millions of people to serve on the front lines, providing logistics to support them, and having unprecedented authority to intervene in the lives of citizens. Fascism views forms of violence—including political violence, imperialist violence, and war—as means to national rejuvenation. Fascists often advocate for the establishment of a totalitarian one-party state, and for a dirigiste economy, which is a market economy in which the state plays a strong directive role through market intervention with the principal goal of achieving national economic self-sufficiency, or "autarky". Fascism emphasizes both palingenesis—national rebirth or regeneration—and modernity when it is deemed compatible with national rebirth. In promoting the nation's regeneration, fascists seek to purge it of decadence. Fascism may also centre around an ingroup-outgroup opposition and demonization of "Others", such as various ethnicities, immigrants, nations, races, political opponents of fascist parties, religious groups, and sexual and gender minorities. In the case of Nazism, this involved racial purity and a belief in a master race. Such demonization has motivated fascist regimes to commit massacres, forced sterilizations, deportations, and genocides. During World War II, the genocidal and imperialist ambitions of the fascist regimes of the Axis powers resulted in the murder of millions of people. Since the end of World War II in 1945, fascism has been largely disgraced, and few parties have openly described themselves as fascist; the term is often used pejoratively by political opponents. The descriptions neo-fascist or post-fascist are sometimes applied to contemporary parties with ideologies similar to, or rooted in, 20th-century fascist movements.

A constitution, or supreme law, is the aggregate of fundamental principles or established precedents that constitute the legal basis of a polity, organization or other type of entity, and commonly determines how that entity is to be governed. When these principles are written down into a single document or set of legal documents, those documents may be said to embody a written constitution; if they are encompassed in a single comprehensive document, it is said to embody a codified constitution. The constitution of the United Kingdom is a notable example of an uncodified constitution; it is instead written in numerous fundamental acts of a legislature, court cases, and treaties. Constitutions concern different levels of organizations, from sovereign countries to companies and unincorporated associations. A treaty that establishes an international organization is also its constitution, in that it would define how that organization is constituted. Within states, a constitution defines the principles upon which the state is based, the procedure in which laws are made, and by whom. Some constitutions, especially codified constitutions, also act as limiters of state power, by establishing lines which a state's rulers cannot cross, such as fundamental rights. Changes to constitutions frequently require consensus or supermajority. The Constitution of India is the longest written constitution of any country in the world, with 146,385 words in its English-language version, while the Constitution of Monaco is the shortest written constitution with 3,814 words. The Constitution of San Marino might be the world's oldest active written constitution, since some of its core documents have been in operation since 1600, while the Constitution of the United States is the oldest active codified constitution. The historical life expectancy of a written constitution since 1789 is approximately 19 years. There are also varieties of constitutions, for example liberal constitutions and communist state constitutions.

A parliament is a type of legislature, or law-making body, of a state. Generally, a parliament has three functions: representing the electorate, making laws, and overseeing the executive government via hearings and inquiries. Its role is similar to that of a senate, synod or congress; a parliament is the institutional form of parliamentary systems based on the fusion of powers. The term parliament is commonly used in countries that are current or former monarchies. Some contexts restrict the use of the word to parliamentary systems, although it is also used to describe the legislature in some presidential systems (e.g., the Parliament of Ghana), even where it is not in the official name. A parliament is typically made up of elected members, who are legislators. Historically, parliaments included various kinds of deliberative, consultative, and judicial assemblies. Parliamentary gatherings in the Middle Ages began to establish that monarchs were subject to law and first summoned representatives of common people, notably the Cortes of León in 1188, and an English parliament in 1265, which developed into an enduring House of Commons. During the early modern period, the Glorious Revolution of 1688 in Britain established the primacy of parliamentary sovereignty, through which the rule of law could be enforced. Many other modern concepts of parliamentary government developed in the Kingdom of Great Britain (1707–1800) and were later exported to other countries around the world. The revolutions of 1848 in Europe largely failed to strengthen parliaments in most European countries, as conservative forces regained control and reversed changes, but they made a return to the pre-1848 status quo impossible, leading to gradual, long-term parliamentary development across much of Europe. Expansion of suffrage in the 19th and 20th centuries led to many parliaments around the world becoming democratically elected.

Capitalism is an economic system based on the private ownership of the means of production and its use for the purpose of obtaining profit. This socioeconomic system has developed historically in several stages and is defined by a number of constituent elements: private property, profit motive, capital accumulation, competitive markets, commodification, wage labor, and an emphasis on innovation and economic growth. Capitalist economies may experience business cycles of economic expansion followed by recessions. Economists, historians, political economists, and sociologists have adopted different perspectives in their analyses of capitalism and have recognized various forms of it in practice. These include laissez-faire capitalism, free-market capitalism, state capitalism, and welfare capitalism. Different forms of capitalism feature varying degrees of free markets, public ownership, barriers to free competition, and state-sanctioned social policies. The degree of competition in markets, the role of intervention and regulation, and the scope of state ownership vary across different models of capitalism. The extent to which markets are free and the rules defining private property are matters of politics and policy. Most of the existing economies in the world today are mixed economies that combine elements of free markets with state intervention and, in some cases, economic planning. Capitalism in its modern form emerged from agrarianism in England, as well as from mercantilist practices in European countries between the 16th and 18th centuries. The Industrial Revolution of the 18th century established capitalism as a dominant mode of production characterized by factory work and a complex division of labor. Through globalization, capitalism spread across the world in the 19th and 20th centuries, especially before World War I and again after the end of the Cold War. During the 19th century, capitalism was largely unregulated by the state but became more regulated in the post–World War II period through Keynesianism, followed by a return of more unregulated capitalism termed neoliberalism starting in the 1980s.

Colonialism is the practice of extending and maintaining political, social, economic, and cultural domination over a territory and its people by another people in pursuit of interests defined in an often distant metropole, who also claim superiority. While frequently an imperialist project, colonialism functions through differentiating between the targeted land and people, and that of the colonizers (a critical component of colonization). Rather than annexation, this typically culminates in organizing the colonized into colonies separate to the colonizers' metropole. Colonialism sometimes deepens by developing settler colonialism, whereby settlers from one or multiple colonizing metropoles occupy a territory with the intention of partially or completely supplanting the existing Indigenous peoples, possibly amounting to genocide. Colonialism monopolizes power by understanding conquered land and people to be inferior, based on beliefs of entitlement and superiority, justified with beliefs of having a civilizing mission to cultivate land and life, historically often rooted in the belief of a Christian mission. These beliefs and the actual colonization establish a so-called coloniality, which keeps the colonized socio-economically othered and subaltern through modern biopolitics of sexuality, gender, race, disability, and class, among others, resulting in intersectional violence and discrimination. While different forms of colonialism have existed around the world, the concept has been developed as a description of European colonial empires of the modern era. These spread globally from the 15th century to the mid-20th century, spanning 35% of Earth's land by 1800 and peaking at 84% by the beginning of World War I. European colonialism employed mercantilism and chartered companies, and established complex colonialities. Decolonization, which started in the 18th century, gradually led to the independence of colonies in waves, with a particular large wave of decolonizations happening in the aftermath of World War II between 1945 and 1975. Colonialism has a persistent impact on a wide range of modern outcomes, as scholars have shown that variations in colonial institutions can account for variations in economic development, regime types, and state capacity. Some academics have used the term neocolonialism to describe the continuation or imposition of elements of colonial rule through indirect means in the contemporary period.

Diplomacy is the communication by representatives of state, intergovernmental, or non-governmental institutions intended to influence events in the international system. Diplomacy is the main instrument of foreign policy which represents the broader goals and strategies that guide a state's interactions with the rest of the world. International treaties, agreements, alliances, and other manifestations of international relations are usually the result of diplomatic negotiations and processes. Diplomats may also help shape a state by advising government officials. Modern diplomatic methods, practices, and principles originated largely from 17th-century European customs. Beginning in the early 20th century, diplomacy became professionalized; the 1961 Vienna Convention on Diplomatic Relations, ratified by most of the world's sovereign states, provides a framework for diplomatic procedures, methods, and conduct. Most diplomacy is now conducted by accredited officials, such as envoys and ambassadors, through a dedicated foreign affairs office. Diplomats operate through diplomatic missions, most commonly consulates and embassies, and rely on a number of support staff; the term diplomat is thus sometimes applied broadly to diplomatic and consular personnel and foreign ministry officials.

The Cold War was a period of international geopolitical rivalry between the United States (US) and the Soviet Union (USSR) and their respective allies, the capitalist Western Bloc and communist Eastern Bloc. It began in the aftermath of the Second World War and ended with the dissolution of the Soviet Union in 1991. The term cold war is used because there was no direct fighting between the two superpowers, though each supported opposing sides in regional conflicts known as proxy wars. In addition to the struggle for ideological and economic influence and an arms race in both conventional and nuclear weapons, the Cold War was expressed through technological rivalries such as the Space Race, espionage, propaganda campaigns, embargoes, and sports diplomacy. After the end of the Second World War in 1945, during which the US and USSR had been allies, the USSR installed satellite governments in its occupied territories in Eastern Europe and North Korea by 1949, resulting in the political division of Europe (and Germany) by an "Iron Curtain". The USSR tested its first nuclear weapon in 1949, four years after their use by the US on Hiroshima and Nagasaki, and allied with the People's Republic of China, founded in 1949. The US declared the Truman Doctrine of "containment" of communism in 1947, launched the Marshall Plan in 1948 to assist Western Europe's economic recovery, and founded the NATO military alliance in 1949 (matched by the Soviet-led Warsaw Pact in 1955). The Berlin Blockade of 1948 to 1949 was an early confrontation, as was the Korean War of 1950 to 1953, which ended in a stalemate. US involvement in regime change during the Cold War included support for First World anti-communist and right-wing dictatorships and uprisings, while Soviet involvement included the funding of Second World left-wing parties, wars of independence, and dictatorships. As nearly all the colonial states underwent decolonization, many became Third World battlefields of the Cold War. Both powers used economic aid in an attempt to win the loyalty of non-aligned countries. The Cuban Revolution of 1959 installed the first communist regime in the Western Hemisphere, and in 1962, the Cuban Missile Crisis began after deployments of US missiles in Europe and Soviet missiles in Cuba; it is widely considered the closest the Cold War came to escalating into nuclear war. Another major proxy conflict was the Vietnam War of 1955 to 1975, which ended in defeat for the US. The USSR solidified its domination of Eastern Europe with its crushing of the Hungarian Revolution in 1956 and the Warsaw Pact invasion of Czechoslovakia in 1968. Relations between the USSR and China broke down by 1961, with the Sino-Soviet split bringing the two states to the brink of war amid a border conflict in 1969. In 1972, the US initiated diplomatic contacts with China and the US and USSR signed a series of treaties limiting their nuclear arsenals during a period known as détente. In 1979, the toppling of US-allied governments in Iran and Nicaragua and the outbreak of the Soviet–Afghan War again raised tensions. In 1985, Mikhail Gorbachev became leader of the USSR and expanded political freedoms, which contributed to the revolutions of 1989 in the Eastern Bloc and the collapse of the USSR in 1991, ending the Cold War.

Anarchism is a political philosophy and movement that seeks to abolish all institutions that perpetuate authority, coercion, or hierarchy, primarily targeting the state and capitalism. Anarchism advocates for the replacement of the state with stateless societies and voluntary free associations. Anarchism is described as being part of the libertarian wing of the socialist movement (libertarian socialism). Although traces of anarchist ideas are found all throughout history, modern anarchism emerged from the Enlightenment. During the latter half of the 19th and the first decades of the 20th century, the anarchist movement flourished in most parts of the world and had a significant role in workers' struggles for emancipation. Various anarchist schools of thought formed during this period. Anarchists have taken part in several revolutions, most notably in the Paris Commune, the Russian Civil War and the Spanish Civil War, whose conclusion marked the end of the classical era of anarchism. In the last decades of the 20th and into the 21st century, the anarchist movement has been resurgent, growing in popularity and influence within anti-capitalist, anti-war and anti-globalisation movements. Anarchists employ a range of approaches to social change, often categorised as revolutionary or evolutionary, though the two frequently overlap and are reflected in a diversity of tactics. Evolutionary approaches generally favour gradual and often nonviolent change, while revolutionary approaches seek to destroy oppressive states and institutions. Many facets of human civilisation have been influenced by anarchist theory, critique, and praxis.

Lost is an American science fiction adventure drama television series created by Jeffrey Lieber, J. J. Abrams, and Damon Lindelof that aired on ABC from September 22, 2004, to May 23, 2010, with a total of 121 episodes over six seasons. It contains elements of supernatural fiction and follows the survivors of Oceanic Airlines flight 815 (flying between Sydney and Los Angeles) after the plane crashes on a mysterious island somewhere in the South Pacific Ocean. Episodes typically feature a primary storyline set on the island, augmented by flashback or flashforward sequences which provide additional insight into the involved characters. Lindelof and Carlton Cuse served as showrunners and were executive producers along with Abrams and Bryan Burk. Inspired by the 2000 film Cast Away, the show is told in a heavily serialized manner. Due to its large ensemble cast and the cost of filming primarily on location in Oahu, Hawaii, the series was one of the most expensive on television, with the pilot alone costing over $14 million. The fictional universe and mythology of Lost were expanded upon by a number of related media—most importantly a series of mini-episodes, called Missing Pieces, and a 12-minute epilogue called "The New Man in Charge". Lost has regularly been ranked by critics as one of the greatest television series of all time. The first season had an estimated average of 16 million viewers per episode on ABC. During the sixth and final season, the show averaged over 11 million U.S. viewers per episode. Nevertheless, the series finale received a strongly polarized reaction from both fans and TV critics alike. Lost was the recipient of hundreds of industry award nominations throughout its run and won numerous of these awards, including the Primetime Emmy Award for Outstanding Drama Series in 2005, Best American Import at the British Academy Television Awards in 2005, the Golden Globe Award for Best Television Series – Drama in 2006, and the Screen Actors Guild Award for Outstanding Performance by an Ensemble in a Drama Series.

Breaking Bad is an American crime drama neo-Western television series created by Vince Gilligan for AMC. Set and filmed in Albuquerque, New Mexico, the series follows Walter White (Bryan Cranston), an overqualified high school chemistry teacher who, after being diagnosed with stage-three lung cancer, begins producing and selling methamphetamine with former student Jesse Pinkman (Aaron Paul) to secure his family's financial future. The series also stars Anna Gunn, Dean Norris, Betsy Brandt, RJ Mitte, Giancarlo Esposito, Bob Odenkirk, and Jonathan Banks. Breaking Bad premiered on AMC on January 20, 2008, and concluded on September 29, 2013, after five seasons and 62 episodes. Its first season received generally positive reviews, while later seasons, especially the fifth and final season, received widespread critical acclaim for the performances, writing, direction, cinematography, and character development. The series had modest viewership during its first three seasons, before viewership increased after it became available on Netflix prior to the fourth season. Viewership rose further during the second half of the final season in 2013, and the series finale became one of the most watched cable television episodes of its era. Since its conclusion, Breaking Bad has been widely regarded by critics as one of the greatest television series of all time. It received numerous accolades, including 16 Primetime Emmy Awards, 2 Golden Globe Awards, 2 Peabody Awards, and a British Academy Television Award. Cranston won the Primetime Emmy Award for Outstanding Lead Actor in a Drama Series four times, Paul won the Primetime Emmy Award for Outstanding Supporting Actor in a Drama Series three times, and Gunn won the Primetime Emmy Award for Outstanding Supporting Actress in a Drama Series twice. The series has continued to appear in retrospective rankings of the best television programs. The series gave rise to the larger Breaking Bad franchise. Better Call Saul, a spin-off series centered on Odenkirk's character Saul Goodman, debuted on AMC on February 8, 2015, and concluded on August 15, 2022. El Camino: A Breaking Bad Movie, a sequel film starring Paul, was released on Netflix and in selected theaters on October 11, 2019.

Chernobyl is a 2019 historical drama television miniseries that revolves around the Chernobyl disaster of 1986 and the cleanup efforts that followed. The series was created and written by Craig Mazin and directed by Johan Renck. It features an ensemble cast led by Jared Harris, Stellan Skarsgård, Emily Watson, and Paul Ritter. The series was produced by HBO in the United States and Sky UK in the United Kingdom. The five-part series premiered simultaneously in the United States on May 6, 2019, and in the United Kingdom on May 7. It received widespread critical acclaim for its performances, historical accuracy, atmosphere, tone, screenplay, cinematography, and musical score. At the 71st Primetime Emmy Awards, it received nineteen nominations and won for Outstanding Limited Series, Outstanding Directing, and Outstanding Writing, while Harris, Skarsgård, and Watson received acting nominations. At the 77th Golden Globe Awards, the series won for Best Miniseries or Television Film and Skarsgård won for Best Supporting Actor in a Series, Miniseries or Television Film. At the 2020 BAFTA TV Awards it won best miniseries and Harris won for Best Actor. The release of each episode was accompanied by a podcast in which Mazin and NPR host Peter Sagal discuss instances of artistic license and the reasoning behind them. While critics, experts and witnesses have noted historical and factual discrepancies in the series, the creators' attention to detail has been widely praised.

A miniseries or mini-series, sometimes called a limited-run series, is a television program that tells a story in a predetermined, limited number of episodes. Many miniseries can also be referred to, and shown, as a television film in several parts. The term "serial" is used in the United Kingdom and in other Commonwealth nations to describe a show that has an ongoing narrative plotline, while "series" is used for a set of episodes in a similar way that "season" is used in North America.

Dexter (stylized in all caps) is an American crime television series that initially aired on Showtime from October 1, 2006, to September 22, 2013. Set in Miami, the series centers on Dexter Morgan (Michael C. Hall), a forensic technician specializing in bloodstain pattern analysis for the fictional Miami Metro Police Department, who leads a secret parallel life as a vigilante serial killer, hunting down murderers inadequately punished by the justice system due to corruption or legal technicalities. The show's first season was derived from the novel Darkly Dreaming Dexter (2004), the first in a series of novels by Jeff Lindsay. It was adapted for television by James Manos Jr., who wrote the pilot episode. The series received mostly positive reviews throughout its run. The show has received multiple awards, including two Golden Globes won by Michael C. Hall and John Lithgow for their roles as Dexter Morgan and Arthur Mitchell, respectively. The fourth season aired its season finale on December 13, 2009, to a record-breaking audience of 2.6 million viewers, making it the most-watched original series episode ever on Showtime at that time. The eighth season originally served as the final season of Dexter ; the season premiere was the most-watched episode of the series, drawing more than 3 million viewers total. The series finale drew 2.8 million viewers, which at the time set a record for the largest audience for an original series finale in Showtime's history. Dexter has continued through an expanded franchise with further series. Two sequel series, Dexter: New Blood and Dexter: Resurrection, have aired from November 2021 to January 2022, and from July 2025, respectively, with Hall reprising the title role and Clyde Phillips as showrunner. A prequel series, Dexter: Original Sin, was released between December 2024 and February 2025, starring Patrick Gibson as Dexter.

Succession is an American satirical black comedy-drama television series created by Jesse Armstrong that aired for four seasons on HBO from June 3, 2018, to May 28, 2023. The series centers on the Roy family, the owners of global media and entertainment conglomerate Waystar RoyCo, and their fight for control of the company amidst uncertainty about the health of the family's patriarch. Brian Cox portrays the family patriarch Logan Roy. His children are played by Alan Ruck as Connor, Jeremy Strong as Kendall, Kieran Culkin as Roman, and Sarah Snook as Shiv. Other starring cast members are Matthew Macfadyen as Tom Wambsgans, Shiv's husband and Waystar executive; Nicholas Braun as Greg Hirsch, Logan's grandnephew also employed by the company; Hiam Abbass as Marcia, Logan's third wife; and Peter Friedman as Frank Vernon, a longtime confidant of Logan; while Dagmara Domińczyk, Arian Moayed, J. Smith-Cameron, Justine Lupe, David Rasche, Fisher Stevens, and Alexander Skarsgård featured in recurring roles before being promoted to the main cast. Succession received universal critical acclaim for its writing, acting, humor, musical score, directing, production values, and examination of its subject matter. Many critics and publications have named the show one of the greatest television series of all time. The series has received several accolades, including three wins each for the Golden Globe for Best Television Series – Drama and the Primetime Emmy for Outstanding Drama Series in 2020, 2022 and 2023 as well as the British Academy Television Award for Best International Programme. Culkin, Cox and Strong each won Golden Globe Award for Best Actor – Television Series Drama for their performances, and Culkin and Strong won the Primetime Emmy Award for Outstanding Lead Actor in a Drama Series. Snook and Macfadyen both also won Emmy Awards: for Lead Actress and Supporting Actor respectively, with Macfadyen winning twice. Armstrong also won four Emmys and a BAFTA for his writing.

Seinfeld ( SYNE-feld) is an American television sitcom created by Larry David and Jerry Seinfeld. It originally aired on NBC from July 5, 1989, to May 14, 1998, with a total of nine seasons consisting of 180 episodes. Its ensemble cast stars Seinfeld as a fictionalized version of himself and focuses on his personal life with his three friends: best friend George Costanza (Jason Alexander), ex-girlfriend Elaine Benes (Julia Louis-Dreyfus), and eccentric apartment neighbor Cosmo Kramer (Michael Richards). Seinfeld is set mostly in and around the titular character's apartment in Manhattan's Upper West Side in New York City. It has been described as "a show about nothing", often focusing on the minutiae of daily life. Interspersed in all episodes of the first seven seasons are moments of stand-up comedy from the fictional Jerry Seinfeld, frequently related to the episode's events. As a rising comedian in the late 1980s, Jerry Seinfeld was presented with an opportunity to create a show with NBC. He asked Larry David, a fellow comedian and friend, to help create a premise for a sitcom. The series was produced by West/Shapiro Productions and Castle Rock Entertainment and is distributed in syndication by Sony Pictures Television. It was largely written by David and Seinfeld along with scriptwriters. A favorite among critics, the series led the Nielsen ratings in Seasons 6 and 9 and finished among the top two (along with ER of the same network) every year from 1994 to 1998. Only two other shows—I Love Lucy and The Andy Griffith Show—finished their runs at the top of the ratings. Seinfeld is universally regarded as one of the greatest and most influential American shows of all time. Its most renowned episodes include "The Chinese Restaurant", "The Soup Nazi", "The Parking Garage", "The Marine Biologist", "The Contest", and "The Strike", which introduced the December 23rd holiday of Festivus. E! named the series the "Number 1 reason [why] the '90s ruled". Quotes from numerous episodes have become catchphrases in popular culture.

The Sopranos is an American psychological crime drama television series created by David Chase for HBO. The series follows Tony Soprano (James Gandolfini), a New Jersey Mafia boss who has panic attacks. He reluctantly begins seeing psychiatrist Dr. Jennifer Melfi (Lorraine Bracco), who encourages him to open up about his difficulties balancing his family life with his criminal life. Other important characters include Tony's family, Mafia colleagues, and rivals, most notably his wife Carmela (Edie Falco), his nephew and protégé Christopher Moltisanti (Michael Imperioli), and his uncle Corrado "Junior" Soprano (Dominic Chianese). Having been greenlit in 1997, the series was broadcast on HBO from January 10, 1999, to June 10, 2007, spanning six seasons and 86 episodes. The show was broadcast as a part of HBO's newly launched Sunday night programing block. After its original HBO run, the series entered broadcast and cable syndication in the United States and other territories. The series was produced by HBO in association with Chase Films and Brad Grey Television. It was primarily filmed at Silvercup Studios in New York City, with some on-location filming in New Jersey. The executive producers throughout the show's run were Chase, Brad Grey, Robin Green, Mitchell Burgess, Ilene S. Landress, Terence Winter, and Matthew Weiner. Widely regarded as one of the greatest and most influential television series of all time, it has been credited by media commentators with ushering in the Second Golden Age of Television, marked by high-quality serialized dramas. The series won multiple awards, including Peabody Awards for its first two seasons, 21 Primetime Emmy Awards, and 5 Golden Globe Awards. The Sopranos has been the subject of extensive critical analysis and debate for its portrayal of organized crime and Italian-American identity, and has been referenced or parodied in other media, including a video game, soundtrack albums and podcasts. It has inspired tie-in merchandise. When the series premiered, several cast members had limited mainstream exposure; many subsequently received major award nominations or appeared in high-profile film and television roles. In March 2018, New Line Cinema announced that they had purchased a film detailing the show's background story, set in the 1960s and 1970s during and after the Newark riots. The film, The Many Saints of Newark (2021), was written by Chase and Lawrence Konner and directed by Alan Taylor. It starred Gandolfini's son Michael Gandolfini as a young Tony Soprano.

Game of Thrones is an American fantasy drama television series created by David Benioff and D. B. Weiss for HBO. It is the first adaptation of the A Song of Ice and Fire franchise, a series of high fantasy novels by George R. R. Martin, the first of which is A Game of Thrones. The show premiered on HBO in the United States on April 17, 2011, and concluded on May 19, 2019, with 73 episodes broadcast over eight seasons. Set on the fictional continents of Westeros and Essos, Game of Thrones has a large ensemble cast and follows several story arcs throughout the course of the show. The first major arc concerns the Iron Throne of the Seven Kingdoms of Westeros through a web of political conflicts among the noble families, either vying to claim the throne or fighting for independence. The second major arc focuses on the last descendant of the realm's deposed ruling dynasty, who has been exiled to Essos and is plotting to return and reclaim the throne. The third follows the Night's Watch, a military order defending the realm against threats from beyond the Seven Kingdoms' northern border. Game of Thrones attracted a record viewership on HBO and has a broad, active, and international fan base. Many critics and publications have named the show one of the greatest television series of all time. Critics have praised the series for its acting, complex characters, story, scope, and production values, although its frequent use of nudity and violence (including sexual violence) generated controversy. The final season received significant criticism for its reduced length and creative decisions, with many considering it a disappointing conclusion. The series received 59 Primetime Emmy Awards, the most by a drama series, including Outstanding Drama Series in 2015, 2016, 2018 and 2019. Its other awards and nominations include three Hugo Awards for Best Dramatic Presentation, a Peabody Award, and five nominations for the Golden Globe Award for Best Television Series – Drama. Two prequel series, House of the Dragon and A Knight of the Seven Kingdoms, premiered on HBO in 2022 and 2026, respectively.

Parks and Recreation (also known as Parks and Rec) is an American political satire mockumentary television sitcom created by Greg Daniels and Michael Schur. The series aired on NBC from April 9, 2009, to February 24, 2015, for 125 episodes, over seven seasons. A special reunion episode aired on April 30, 2020. The series stars Amy Poehler as Leslie Knope, a perky, mid-level bureaucrat in the Parks and Recreation Department of the fictional town of Pawnee, Indiana. The ensemble and supporting cast features Rashida Jones as Ann Perkins, Aziz Ansari as Tom Haverford, Nick Offerman as Ron Swanson, Aubrey Plaza as April Ludgate, Chris Pratt as Andy Dwyer, Paul Schneider as Mark Brendanawicz, Adam Scott as Ben Wyatt, Rob Lowe as Chris Traeger, Jim O'Heir as Garry "Jerry" Gergich, Retta as Donna Meagle, and Billy Eichner as Craig Middlebrooks. The writers researched local California politics for the series and consulted with urban planners and elected officials. Leslie Knope underwent major changes after the first season, in response to audience feedback that the character seemed unintelligent and "ditzy". The writing staff incorporated current events into the episodes, such as a government shutdown in Pawnee inspired by the 2008 financial crisis. Real-life political figures, including John McCain, Michelle Obama, Newt Gingrich, and Joe Biden, have cameos in later episodes. Parks and Recreation was part of NBC's "Comedy Night Done Right" programming during its Thursday night prime-time block. The series received mixed reviews during its first season (including comparisons to The Office, a sitcom also produced by Daniels and Schur), but, after a re-approach to its tone and format, the second and subsequent seasons were widely acclaimed. Parks and Recreation received several awards and nominations, including 14 Primetime Emmy Award nominations (two for Outstanding Comedy Series), a Golden Globe Award win for Poehler's performance and a nomination for the Golden Globe Award for Best Television Series – Musical or Comedy, and a Peabody Award. In Time's 2012 year-end lists issue, Parks and Recreation was named the number one television series of that year. In 2013, after receiving four consecutive nominations in the category, Parks and Recreation won the Television Critics Association Award for Outstanding Achievement in Comedy.

24 is an American action drama television series created by Joel Surnow and Robert Cochran for Fox. The series features an ensemble cast, with Kiefer Sutherland starring as American counter-terrorist federal agent Jack Bauer. Each season covers 24 consecutive hours using the real time method of narration, which is emphasized by the display of split screens and a digital clock. Multiple ongoing plot lines of intersecting relevance are covered, with Bauer's plot line serving as the link throughout. The show premiered on November 6, 2001, and spanned 204 episodes over nine seasons, with the series finale broadcast on July 14, 2014. In addition, the television film 24: Redemption aired between seasons six and seven, on November 23, 2008. 24 is a joint production by Imagine Television and 20th Century Fox Television. Bauer is portrayed as a highly proficient agent with an "ends justify the means" approach, willing to lie, threaten, or even harm anyone who refuses to cooperate with him. Throughout the series, the plot elements contain both a political thriller and serial drama, with each episode typically ending on a cliffhanger. Bauer uses people on both sides of the law in his attempts to prevent terrorist attacks and bring down those responsible, sometimes at great personal expense. These attacks include presidential assassination attempts, bomb detonations, bioterrorism and cyberwarfare, as well as conspiracies that involve government and corporate corruption. 24 received generally positive reviews, with the fifth season being universally praised by critics. The show won numerous awards throughout its run, including Best Drama Series at the 2004 Golden Globe Awards and Outstanding Drama Series at the 2006 Primetime Emmy Awards. In May 2013, it was announced that 24 would return with a 12-episode limited series titled 24: Live Another Day, which aired from May 5 to July 14, 2014, bringing the episode count to 204. A spin-off series, 24: Legacy, premiered on February 5, 2017, lasting a single 12-episode season. 24 is the longest-running American espionage or counterterrorism-themed television drama, surpassing both Mission: Impossible and Britain's The Avengers.

The Mandalorian, also known as Star Wars: The Mandalorian, is an American space Western television series created by Jon Favreau for the streaming service Disney+. It is the first live-action series in the Star Wars franchise and begins five years after the events of the film Return of the Jedi (1983). It follows a lone bounty hunter who protects a Force-sensitive child, Grogu, from remnant Imperial forces. Pedro Pascal stars as the title character, with Katee Sackhoff co-starring in the third season. Star Wars creator George Lucas had begun developing a live-action Star Wars television series by 2009, but it was deemed too expensive to produce. He sold Lucasfilm to Disney in October 2012, and work on a new Star Wars series began for Disney+. Favreau signed on as writer and showrunner in March 2018. He executive produces alongside Dave Filoni, Kathleen Kennedy, and Colin Wilson; Rick Famuyiwa joined them for the third season. The title was announced in October 2018 when filming started at Manhattan Beach Studios in California. Visual effects company Industrial Light & Magic developed the StageCraft technology for the series, displaying digital backgrounds on a 360-degree video wall. This has since been adopted by other film and television productions. The Mandalorian premiered with the launch of Disney+ on November 12, 2019. The rest of the first season was released through December 27. A second season was released from October to December 2020, and a third season was released from March to April 2023. The series has received largely positive reviews from critics and several accolades, including Primetime Creative Arts Emmy Award wins for all three seasons. A fourth season of the series had been in development, before creative plans changed and development shifted towards a feature film which became The Mandalorian and Grogu; the film acts as a continuation of the series, was directed by Favreau, and was released in May 2026. Interconnected spin-off series The Book of Boba Fett, Ahsoka, and Skeleton Crew expand on The Mandalorian's timeframe, with an untitled feature film directed by Filoni set to serve as a conclusion to the interconnected stories.

Stranger Things is an American television series created by the Duffer Brothers for Netflix. Produced by Monkey Massacre Productions and 21 Laps Entertainment, the first season was released on Netflix on July 15, 2016. The second and third seasons followed in October 2017 and July 2019, respectively, and the fourth season was released in two volumes in May and July 2022. The fifth and final season was released in three volumes, including the finale, in November and December 2025. The show combines elements of horror, science fiction, mystery, coming-of-age, and drama. Set in the 1980s, the series centers on the residents of the fictional town of Hawkins, Indiana, after a young girl with psychokinetic abilities named Eleven inadvertently creates a wormhole known as the Upside Down at a nearby secretive government research facility, connecting Earth to a hostile realm called the Abyss. The ensemble cast includes Winona Ryder, David Harbour, Finn Wolfhard, Millie Bobby Brown, Gaten Matarazzo, Caleb McLaughlin, Natalia Dyer, Charlie Heaton, Cara Buono, Matthew Modine, Noah Schnapp, Sadie Sink, Joe Keery, Dacre Montgomery, Sean Astin, Paul Reiser, Maya Hawke, Priah Ferguson, Brett Gelman, Linda Hamilton, Jamie Campbell Bower, and Nell Fisher. The Duffer Brothers developed Stranger Things as a blend of investigative drama and supernatural horror, infused with childlike wonder and references to the popular culture of the 1980s. Several thematic and directorial elements were inspired by the works of Steven Spielberg, John Carpenter, David Lynch, Stephen King, Wes Craven, H. P. Lovecraft, and FromSoftware. The show also drew inspiration from Cold War-era experiments and conspiracy theories involving secret government programs. Stranger Things has received positive reviews throughout its run, with praise for its characterization, atmosphere, acting, direction, writing, and homage to 1980s cinema, making it a touchstone of 1980s nostalgia. It has garnered many accolades and is widely regarded as one of the greatest television shows of all time. Stranger Things is a flagship series for Netflix, drawing record viewership with each season's release. The series has expanded into a media franchise, including an animated spin-off, Tales from '85 (2026), and a prequel stage play, The First Shadow (2023). It has also inspired several books, comics, tie-in media, a pop-up shop, and a Dungeons & Dragons role-playing starter set game based on the series.

Dark is a German science-fiction mystery thriller television series created by Baran bo Odar and Jantje Friese. It ran for three seasons from 2017 to 2020. The story primarily follows four families from the fictional town of Winden, Germany, as they pursue the truth in the aftermath of a child's disappearance, unraveling a sinister time travel conspiracy that spans several generations. The series explores the existential implications of time and its effect on human nature and life, with its plot structure making prominent use of the bootstrap paradox. It features a large ensemble cast led by Louis Hofmann. Dark debuted on 1 December 2017 on Netflix; it is the service's first German-language original series. The second season was released on 21 June 2019, while the third and final season was released on 27 June 2020. Dark has received critical acclaim for its performances, casting, direction, writing, tone, visuals, themes, musical score, and the ambition and complexity of its narrative structure. It has been nominated for and won several awards. In 2021, the BBC ranked the series as the 58th greatest TV series of the 21st century.

The Simpsons is an American animated sitcom created by Matt Groening and developed by Groening, James L. Brooks and Sam Simon for the Fox Broadcasting Company. It is a satirical depiction of American life, epitomized by the Simpson family, which consists of Homer, Marge, Bart, Lisa, and Maggie. Set in the fictional town of Springfield, in an unspecified location in the United States, it caricatures society, Western culture, television, and the human condition. Widely regarded as one of the most influential animated series of all time, The Simpsons has been named by Time as the greatest television series of the 20th century. The family was conceived by Groening shortly before a solicitation for a series of animated shorts with producer Brooks. He created a dysfunctional family and named the characters after his own family members, substituting Bart for his own name; he thought Simpson was a funny name in that it sounded similar to "simpleton". The shorts became a part of The Tracey Ullman Show on April 19, 1987. After three seasons, the sketch was developed into a half-hour primetime show and became Fox's first series to land in the Top 30 ratings in a season (1989–1990). Since its debut on December 17, 1989, 805 episodes of the show have been broadcast. It is the longest-running American animated series, longest-running American sitcom, and the longest-running American scripted primetime television series, both in seasons and individual episodes. A feature-length film, The Simpsons Movie (2007), was released in theaters worldwide to critical and commercial success, with a sequel set to release in 2027. The series has also spawned numerous comic book series, video games, books and other related media, as well as a billion-dollar merchandising industry. The Simpsons was initially a joint production by Gracie Films and 20th Television; 20th Television's involvement was later moved to 20th Television Animation, a separate unit of Disney Television Studios. On April 2, 2025, the show was renewed for four additional seasons on Fox, with 15 episodes and two Disney+ specials each, bringing the episode total from 790 to 858. The Simpsons received widespread critical acclaim throughout its early seasons in the 1990s, which are generally considered its "golden age". Since then, it has been criticized for a perceived decline in quality. Time named it the 20th century's best television series while Erik Adams of The A.V. Club named it "television's crowning achievement regardless of format". On January 14, 2000, the Simpson family was awarded a star on the Hollywood Walk of Fame. It has won dozens of awards since it debuted as a series, including 37 Primetime Emmy Awards, 34 Annie Awards, and 2 Peabody Awards. Homer's exclamatory catchphrase of "D'oh!" has been adopted into the English language. Upon its debut, The Simpsons was often credited with changing comedy on television and is noted for inspiring many other later adult-oriented, popular animated sitcom television series.

The Wire is an American crime drama television series created and primarily written by American author and former police reporter David Simon for the cable network HBO. The series premiered on June 2, 2002, and ended on March 9, 2008, comprising 60 episodes over five seasons. The idea for the show started out as a police drama loosely based on the experiences of Simon's writing partner Ed Burns, a former homicide detective and public school teacher. Set and produced in Baltimore, Maryland, The Wire introduces a different institution of the city and its relationship to law enforcement in each season while retaining characters and advancing storylines from previous seasons. The five subjects are, in chronological order: the illegal drug trade, the port system, the city government and bureaucracy, education and schools, and the print news medium. Simon chose to set the show in Baltimore because of his familiarity with the city. When the series first aired, the large cast consisted mainly of actors who were unknown to television audiences, as well as numerous real-life Baltimore and Maryland figures in guest and recurring roles. Simon has said that despite its framing as a crime drama, the show is "really about the American city, and about how we live together. It's about how institutions have an effect on individuals. Whether one is a cop, a longshoreman, a drug dealer, a politician, a judge or a lawyer, all are ultimately compromised and must contend with whatever institution to which they are committed". Lauded for high quality scripting and an uncommonly accurate exploration of society, politics and urban life, the series received average ratings and failed to win any major television awards when originally broadcast. Subsequently, the show gained a cult following, and is now widely considered one of the greatest television series of all time.

The X-Files is an American science fiction drama television series created by Chris Carter. The original series aired from September 10, 1993, to May 19, 2002, on Fox, spanning nine seasons, with 202 episodes. A tenth season of six episodes ran from January to February 2016. Following the ratings success of this revival, The X-Files returned for an eleventh season of ten episodes, which ran from January to March 2018. In addition to the television series, two feature films have been released: the 1998 film The X-Files and the stand-alone film The X-Files: I Want to Believe, released in 2008, six years after the original television run ended. The series revolves around Federal Bureau of Investigation (FBI) Special Agents Fox Mulder (David Duchovny) and Dana Scully (Gillian Anderson), who investigate the eponymous "X-Files": marginalized, unsolved cases involving paranormal phenomena. Mulder is a skilled criminal profiler, an ardent supernaturalist, and a conspiracy theorist who believes in the existence of the paranormal, whereas Scully is a medical doctor and skeptic who has been assigned to scientifically analyze Mulder's case files. Early in the series, both agents apparently become pawns in a much larger conflict and come to trust only each other and select others. The agents discover what appears to be a governmental agenda to hide evidence of extraterrestrial life. Mulder and Scully's shared adventures initially lead them to develop a close platonic bond, which develops into a complex romantic relationship. Roughly one third of the series' episodes follow a complicated mythopoeia-driven story arc about a planned alien invasion, whereas the other two-thirds may be described as "monster of the week" episodes that focus on a single villain, mutant, or monster. The X-Files was inspired by earlier television series featuring elements of suspense, horror, and speculative science fiction, including The Twilight Zone, Night Gallery, Tales from the Darkside, Twin Peaks, and especially Kolchak: The Night Stalker. When creating the main characters, Carter sought to reverse gender stereotypes by making Mulder a believer and Scully a skeptic. The first seven seasons featured Duchovny and Anderson relatively equally. In the eighth and ninth seasons, Anderson took precedence while Duchovny appeared intermittently. New main characters were introduced: FBI Special Agents John Doggett (Robert Patrick) and Monica Reyes (Annabeth Gish), among others. Mulder and Scully's immediate superior, Assistant Director Walter Skinner (Mitch Pileggi), began to appear regularly. The first five seasons of The X-Files were filmed in Vancouver, British Columbia, before production eventually moved to Los Angeles, apparently to accommodate Duchovny's schedule. However, the series later returned to Vancouver with the filming of The X-Files: I Want to Believe as well as the tenth and eleventh seasons. The X-Files was a hit for the Fox network and received largely positive reviews, although its long-term story arc was criticized near the conclusion. Initially considered a cult series, it turned into a pop culture touchstone that tapped into public mistrust of governments and large institutions and embraced conspiracy theories and spirituality. Both the series and lead actors Duchovny and Anderson received multiple awards and nominations, and by its conclusion the show was the longest-running science fiction series in American television history. The series also spawned a franchise that includes spin-offs Millennium and The Lone Gunmen, two theatrical films, and accompanying merchandise.

The Office is an American mockumentary sitcom television series. It is based on the BBC series The Office created by Ricky Gervais and Stephen Merchant, and adapted for NBC by Greg Daniels. The show depicts the everyday work lives of office employees at the Scranton, Pennsylvania branch of the fictional Dunder Mifflin Paper Company. It aired from March 24, 2005, to May 16, 2013, for a total of nine seasons consisting of 201 episodes. The show was co-produced by Daniels' Deedle-Dee Productions, Reveille Productions (later Shine America) and 3 Arts Entertainment (although uncredited) in association with Universal Television. The original executive producers were Daniels, Gervais, Merchant, Howard Klein and Ben Silverman, with numerous others being promoted in later seasons. Like the British original (which aired from 2001 to 2003), the series was filmed in a single-camera setup without a studio audience or a laugh track, to mirror the look of an actual documentary. It debuted on NBC as a mid-season replacement and ended its nine-season run on May 16, 2013, with a two-part series finale. Its original main cast was Steve Carell, Rainn Wilson, John Krasinski, Jenna Fischer, and B. J. Novak. It experienced numerous changes to its ensemble cast during its run. Stars outside the original main cast include Ed Helms, Rashida Jones, Amy Ryan, Mindy Kaling, Craig Robinson, James Spader, Ellie Kemper, and Catherine Tate. The Office received moderately positive reviews from critics (except for the pilot episode, which received mixed reviews) during its relatively short first season of 6 episodes, a British television standard, but the following seasons, particularly Carell's performance, received significant acclaim from television critics as the show's characters, content, structure, and tone diverged considerably from the original British series. These seasons were included on several critics' year-end top TV series lists, winning several awards including a Peabody Award in 2006, two Screen Actors Guild Awards, a Golden Globe Award for Carell's performance, and five Primetime Emmy Awards, including one for Outstanding Comedy Series, in 2006. The eighth season was criticized for declining quality, with Carell's departure in season seven seen as a contributing factor. However, the ninth and final season ended the series with a generally positive response. The series finale, which originally aired on May 16, 2013, was viewed by an estimated 5.7 million viewers and garnered critical acclaim. In 2016, Rolling Stone named The Office one of the 100 greatest television shows of all time.

Fargo is an American black comedy crime drama television series created by Noah Hawley for FX. It is based on the 1996 film written and directed by Joel and Ethan Coen. The Coens, whose other films also heavily influenced the series, were originally uninvolved with the series, but joined as executive producers after being impressed by Hawley's script for the first season. The series premiered on April 15, 2014, on FX. Fargo is an anthology series consisting of self-contained seasons, although they all share the same continuity as the film and feature minor overlaps. Five seasons of the show have been released between 2014 and 2024, each with a different setting, cast and characters; the time periods also largely vary, ranging from 1950 in season four to 2019 in season five, although they all take place in the American Midwest, primarily Minnesota. The storylines differ with each season, but typically follow large casts of characters including organized criminals, law enforcement and regular civilians as their interwoven lives spiral into conflicts and bloodshed. The series received a large number of awards and nominations, mostly for its performances; these include seven Primetime Emmy Awards (including Outstanding Limited Series for its first season) out of 70 nominations, three Golden Globe Awards (including Best Miniseries or Television Film, also for its first season) out of 14 nominations, and eight Critics' Choice Television Awards (including Best Movie/Miniseries twice for its first two seasons) out of 22 nominations.

Black Mirror is a British anthology television series created by Charlie Brooker. Most episodes are speculative fiction, set in near-future dystopias containing sci-fi technology. The series is inspired by The Twilight Zone and uses the themes of technology and media to comment on contemporary social issues. Most episodes are written by Brooker with involvement by the executive producer Annabel Jones. There are 33 episodes in seven series and one special, in addition to the interactive film Black Mirror: Bandersnatch (2018). The first two series aired on the British network Channel 4 in 2011 and 2013, as did the 2014 special "White Christmas". The programme then moved to Netflix, where five further series aired in 2016, 2017, 2019, 2023, and 2025. An eighth series is in development. Two related webisode series were produced by Netflix, and a companion book to the first four series, Inside Black Mirror, was published in 2018. Soundtracks to many episodes have been released as albums. Black Mirror is considered by some reviewers to be one of the best television series of the 2010s, while some critics have found the formulaic morality themes of the series obvious or have cited declining quality. The programme won the Primetime Emmy Award for Outstanding Television Movie three times consecutively for "San Junipero", "USS Callister" and Bandersnatch. Black Mirror, along with American Horror Story and Inside No. 9, has been credited with reviving the anthology television format and a number of episodes have been deemed prescient by the media.

The Crown is a historical drama television series about the reign of Queen Elizabeth II, created and principally written by Peter Morgan and produced by Left Bank Pictures and Sony Pictures Television for Netflix. Morgan developed the series from his film The Queen (2006) and his stage play The Audience (2013), which also focused on Elizabeth. The series consists of six seasons spanning almost six decades, beginning shortly before the wedding of Princess Elizabeth and Philip Mountbatten in 1947 and ending with the 2005 wedding of Prince Charles and Camilla Parker Bowles. The principal cast of the series has been changed every two seasons; Elizabeth was played by Claire Foy in the first and second seasons, Olivia Colman in the third and fourth, and Imelda Staunton in the fifth and sixth. Filming took place at Elstree Studios in Borehamwood, Hertfordshire, with location shooting taking place in the United Kingdom and internationally. Netflix released the first season on 4 November 2016; the sixth was released in two parts, the first on 16 November 2023 and the second on 14 December 2023. The Crown has been praised by critics for its acting, directing, writing, cinematography, and production value. However, its historical inaccuracies have been criticised, particularly in the latter half of the series's run. The series has won numerous awards, including a Primetime Emmy Award for Outstanding Drama Series and two Golden Globe Awards for Best Television Series-Drama.

The Peaky Blinders were a street gang based in Birmingham, England, which operated from the 1880s until the 1920s. The group consisted largely of young criminals from lower- to working-class backgrounds. They engaged in robbery, violence, racketeering, illegal bookmaking, and control of gambling. By the 20th century members wore signature outfits that typically included tailored jackets, lapelled overcoats, buttoned waistcoats, silk scarves, bell-bottom trousers, leather boots, and flat caps. The Blinders' dominance came about from beating rivals, including the "Sloggers", "a pugilistic term for someone who could strike a heavy blow in the ring", whom they fought for territory in Birmingham and its surrounding districts. They held "control" for 30 years until 1920, when a larger gang, the Birmingham Boys, led by Billy Kimber, overtook them. Although they had disappeared by the 1920s, the name "Peaky Blinders" became synonymous slang for any street gang in Birmingham. In 2013, the name was reused for a BBC Television series entitled Peaky Blinders. The series, which stars Cillian Murphy, Paul Anderson, Helen McCrory, Sophie Rundle and Joe Cole, is a crime story about a fictional crime family operating in Birmingham just after World War I.

Sherlock is a British mystery crime drama television series based on Sir Arthur Conan Doyle's Sherlock Holmes detective stories. Created by Steven Moffat and Mark Gatiss, the show stars Benedict Cumberbatch as Sherlock Holmes and Martin Freeman as Doctor John Watson. Thirteen episodes were produced, with four three-part series airing from 2010 to 2017 and a special episode that aired on 1 January 2016. The series is set in the present day. The one-off special features a Victorian-period fantasy resembling the original Holmes stories. Sherlock was produced by the BBC, along with Hartswood Films; Moffat, Gatiss, Sue Vertue and Rebecca Eaton were the executive producers. The series was supported by the American station WGBH-TV Boston for its PBS anthology series, Masterpiece, where it also aired in the United States. The series was filmed primarily in Cardiff. North Gower Street in London was used for exterior shots of Holmes's and Watson's 221B Baker Street residence. Sherlock's first three series received praise for the quality of the writing, acting, and directing, though the final series had a mixed reception. The programme has been nominated for numerous awards including Emmys, BAFTAs and a Golden Globe, winning several awards across a variety of categories prior to its final series. The show won in three categories at the 66th Primetime Emmy Awards including Outstanding Lead Actor in a Miniseries or a Movie for Cumberbatch, Outstanding Supporting Actor in a Miniseries or a Movie for Freeman, and Outstanding Writing for a Miniseries, Movie or a Dramatic Special for Moffat. Two years later, it won Outstanding Television Movie. In addition, the show was also honoured with a Peabody Award in 2011. The third series became the UK's most watched drama series since 2001. Sherlock has been sold to 180 territories. All of the series have been released on DVD and Blu-ray, alongside tie-in editions of selected original Conan Doyle books and an original soundtrack composed by David Arnold and Michael Price. In January 2014, the show launched its official mobile app called Sherlock: The Network.

Arrested Development is an American satirical television sitcom created by Mitchell Hurwitz that debuted on Fox on November 2, 2003. It follows the Bluth family, a formerly wealthy, dysfunctional family and is presented in a serialized format, incorporating handheld camera work, voice-over narration, archival photos and historical footage, and maintains numerous running gags and catchphrases. Ron Howard served as both an executive producer and the omniscient narrator and, in later seasons, appears in the show as a fictionalized version of himself. Set in Newport Beach, California, the series was filmed primarily in Culver City and Marina del Rey. Arrested Development received critical acclaim. It won six Primetime Emmy Awards and a Golden Globe Award, and attracted a cult following. It has been widely regarded as one of the greatest TV shows of all time. It influenced later single-camera comedy series such as 30 Rock and Community. Despite the positive critical response, Arrested Development received low ratings on Fox, which canceled the series in 2006. In 2011, Netflix licensed new episodes and distributed them on its streaming service. These episodes were released in May 2013, and was among the first of Netflix's original programming. Netflix commissioned a fifth season of Arrested Development, the first half of which premiered in May 2018, and the second half in March 2019.

Band of Brothers is a 2001 American war drama miniseries based on historian Stephen E. Ambrose's 1992 non-fiction book of the same name. It was created by Steven Spielberg and Tom Hanks, who also served as executive producers, and who had collaborated on the 1998 World War II film Saving Private Ryan, from which the series got many of its visual cues and crew members. Episodes first aired on HBO from September 9 to November 4, 2001. The series dramatizes the history of "Easy" Company, 2nd Battalion, 506th Parachute Infantry Regiment of the 101st Airborne Division. It begins during Easy Company's paratrooper training and follows its participation in the Western Front of World War II from D-Day to their occupation of Berchtesgaden. The events are based on Ambrose's research and recorded interviews with Easy Company veterans. Although all the characters are based directly on members of Easy Company, the series took some literary license, adapting history for dramatic effect and series structure. Each episode begins with excerpts from interviews with some of the survivors, who are identified by name only at the end of the finale. The title of the book and series comes from the St. Crispin's Day speech in William Shakespeare's play Henry V, delivered by King Henry before the Battle of Agincourt. Ambrose quotes a passage from the speech on his book's first page; this passage is recited by Carwood Lipton in the series finale. Band of Brothers received universal acclaim and would go on to win the Primetime Emmy Award for Outstanding Miniseries. Retrospective reviews have cited it as one of the greatest television shows of all time, and it is widely seen as a pioneering entry in Peak TV in large part due to its high production value which many compared favorably to Saving Private Ryan. Its success led to the creation of two companion piece miniseries, also with Spielberg's and Hanks' involvement, that feature the exploits of other military branches during World War II: The Pacific (2010) and Masters of the Air (2024).