Establishments – Disestablishments
The 20th (twentieth) century began on January 1, 1901 ( MCMI), and ended on December 31, 2000 ( MM).  The 20th century was dominated by significant events that defined the era: Spanish flu pandemic, World War I and World War II, nuclear weapons, nuclear power and space exploration, nationalism and decolonization, technological advances, and the Cold War and post-Cold War conflicts.
The 20th century saw a massive transformation of the world order: global total fertility rates, sea level rise and ecological collapses increased; the resulting competition for land and dwindling resources accelerated deforestation, water depletion, and the mass extinction of many of the world's species and decline in the population of others; consequences which are now being dealt with. Man-made global warming increased the risk of extreme weather conditions.
Additional themes include intergovernmental organizations and cultural homogenization through developments in emerging transportation and communications technology; poverty reduction and world population growth, awareness of environmental degradation, ecological extinction;   and the birth of the Digital Revolution. Automobiles, airplanes and the use of home appliances became common, as did video and audio recording. Great advances in power generation, communication, and medical technology allowed for near-instantaneous worldwide computer communication and genetic modification of life.
The repercussions of the World Wars, Cold War, and globalization crafted a world where people are more united than any previous time in human history, as exemplified by the establishment of international law, international aid, and the United Nations. The Marshall Plan—which spent $13 billion ($100 billion in 2019 U.S. dollars)  to rebuild the economies of post-war nations—launched " Pax Americana". Throughout the latter half of the 20th century, the rivalry between the United States and the Soviet Union created enormous tensions around the world which manifested in various armed proxy regional conflicts and the omnipresent danger of nuclear proliferation. The dissolution of the Soviet Union in 1991 after the collapse of its European alliance was heralded by the West as the end of communism, though by the century's end roughly one in six people on Earth lived under communist rule, mostly in China which was rapidly rising as an economic and geopolitical power.
It took over two hundred thousand years of modern human history and 6 million years of human evolution for the world's population to reach 1 billion in 1804;  world population reached an estimated 2 billion in 1927; by late 1999, the global population reached 6 billion, with over half in East, South and Southeast Asia.    Global literacy averaged 80%. Penicillin and other medical breakthroughs combined with the World Health Organization's global campaigns to eradicate smallpox and other diseases responsible for more human deaths than all wars and natural disasters combined yielded unprecedented results; smallpox now only existed in labs.  Machines came to be used in all areas of production, feeding increasingly intricate supply chains that allowed mankind for the first time to be constrained not by how much it could produce, but by peoples' willingness to consume. Trade improvements greatly expanded upon the limited set of food-producing techniques used since the Neolithic period, multiplying the diversity of foods available and boosting the quality of human nutrition. Until the early 19th century, life expectancy from birth was about thirty in most populations; global lifespan-averages exceeded 40 years for the first time in history, with over half achieving 70 or more years (three decades longer than a century earlier). 
The 20th (twentieth) century began on January 1, 1901,  and ended on December 31, 2000.  The term is often used erroneously to refer to "the 1900s", the century between January 1, 1900 and December 31, 1999. It was the tenth and final century of the 2nd millennium. Unlike most century years, the year 2000 was a leap year, and the second century leap year in the Gregorian calendar after 1600.
The century had the first global-scale total wars between world powers across continents and oceans in World War I and World War II. Nationalism became a major political issue in the world in the 20th century, acknowledged in international law along with the right of nations to self-determination, official decolonization in the mid-century, and related regional conflicts.
The century saw a major shift in the way that many people lived, with changes in politics, ideology, economics, society, culture, science, technology, and medicine. The 20th century may have seen more technological and scientific progress than all the other centuries combined since the dawn of civilization. Terms like nationalism, globalism, environmentalism, ideology, world war, genocide, and nuclear war entered common usage. Scientific discoveries, such as the theory of relativity and quantum physics, profoundly changed the foundational models of physical science, forcing scientists to realize that the universe was more complex than previously believed, and dashing the hopes (or fears) at the end of the 19th century that the last few details of scientific knowledge were about to be filled in. It was a century that started with horses, simple automobiles, and freighters but ended with high-speed rail, cruise ships, global commercial air travel and the Space Shuttle. Horses and other pack animals, every society's basic form of personal transportation for thousands of years, were replaced by automobiles and buses within a few decades. These developments were made possible by the exploitation of fossil fuel resources, which offered energy in an easily portable form, but also caused concern about pollution and long-term impact on the environment. Humans explored space for the first time, taking their first footsteps on the Moon.
Mass media, telecommunications, and information technology (especially computers, paperback books, public education, and the Internet) made the world's knowledge more widely available. Advancements in medical technology also improved the health of many people: the global life expectancy increased from 35 years to 65 years. Rapid technological advancements, however, also allowed warfare to reach unprecedented levels of destruction. World War II alone killed over 60 million people, while nuclear weapons gave humankind the means to annihilate itself in a short time. However, these same wars resulted in the destruction of the imperial system. For the first time in human history, empires and their wars of expansion and colonization ceased to be a factor in international affairs, resulting in a far more globalized and cooperative world. The last time major powers clashed openly was in 1945, and since then, violence has seen an unprecedented decline. 
The world also became more culturally homogenized than ever with developments in transportation and communications technology, popular music and other influences of Western culture, international corporations, and what was arguably a truly global economy by the end of the 20th century.
Technological advancements during World War I changed the way war was fought, as new inventions such as tanks, chemical weapons, and aircraft modified tactics and strategy. After more than four years of trench warfare in Western Europe, and 20 million dead, the powers that had formed the Triple Entente ( France, Britain, and Russia, later replaced by the United States and joined by Italy and Romania) emerged victorious over the Central Powers ( Germany, Austria-Hungary, the Ottoman Empire and Bulgaria). In addition to annexing many of the colonial possessions of the vanquished states, the Triple Entente exacted punitive restitution payments from them, plunging Germany in particular into economic depression. The Austro-Hungarian and Ottoman empires were dismantled at the war's conclusion. The Russian Revolution resulted in the overthrow of the Tsarist regime of Nicholas II and the onset of the Russian Civil War. The victorious Bolsheviks then established the Soviet Union, the world's first communist state.
At the beginning of the period, the British Empire was the world's most powerful nation,  having acted as the world's policeman for the past century. Fascism, a movement which grew out of post-war angst and which accelerated during the Great Depression of the 1930s, gained momentum in Italy, Germany, and Spain in the 1920s and 1930s, culminating in World War II, sparked by Nazi Germany's aggressive expansion at the expense of its neighbors. Meanwhile, Japan had rapidly transformed itself into a technologically advanced industrial power and, along with Germany and Italy, formed the Axis powers. Japan's military expansionism in East Asia and the Pacific Ocean brought it into conflict with the United States, culminating in a surprise attack which drew the US into World War II. After some years of dramatic military success, Germany was defeated in 1945, having been invaded by the Soviet Union and Poland from the East and by the United States, the United Kingdom, Canada, and France from the West. After the victory of the Allies in Europe, the war in Asia ended with the Soviet invasion of Manchuria and the dropping of two atomic bombs on Japan by the US, the first nation to develop nuclear weapons and the only one to use them in warfare. In total, World War II left some 60 million people dead. After the war, Germany was occupied and divided between the Western powers and the Soviet Union. East Germany and the rest of Eastern Europe became Soviet puppet states under communist rule. Western Europe was rebuilt with the aid of the American Marshall Plan, resulting in a major post-war economic boom, and many of the affected nations became close allies of the United States.
With the Axis defeated and Britain and France rebuilding, the United States and the Soviet Union were left standing as the world's only superpowers. Allies during the war, they soon became hostile to one another as their competing ideologies of communism and democratic capitalism proliferated in Europe, which became divided by the Iron Curtain and the Berlin Wall. They formed competing military alliances ( NATO and the Warsaw Pact) which engaged in a decades-long standoff known as the Cold War. The period was marked by a new arms race as the USSR became the second nation to develop nuclear weapons, which were produced by both sides in sufficient numbers to end most human life on the planet had a large-scale nuclear exchange ever occurred. Mutually assured destruction is credited by many historians as having prevented such an exchange, each side being unable to strike first at the other without ensuring an equally devastating retaliatory strike. Unable to engage one another directly, the conflict played out in a series of proxy wars around the world—particularly in China, Korea, Cuba, Vietnam, and Afghanistan—as the USSR sought to export communism while the US attempted to contain it. The technological competition between the two sides led to substantial investment in research and development which produced innovations that reached far beyond the battlefield, such as space exploration and the Internet.
In the latter half of the century, most of the European-colonized world in Africa and Asia gained independence in a process of decolonization. Meanwhile, globalization opened the door for several nations to exert a strong influence over many world affairs. The US's global military presence spread American culture around the world with the advent of the Hollywood motion picture industry, Broadway, rock and roll, pop music, fast food, big-box stores, and the hip-hop lifestyle. Britain also continued to influence world culture, including the " British Invasion" into American music, leading many rock bands from other countries (such as Swedish ABBA) to sing in English. After the Soviet Union collapsed under internal pressure in 1991, most of the communist governments it had supported around the world were dismantled—with the notable exceptions of China, North Korea, Cuba, Vietnam, and Laos—followed by awkward transitions into market economies.
Following World War II, the United Nations, successor to the League of Nations, was established as an international forum in which the world's nations could discuss issues diplomatically. It enacted resolutions on such topics as the conduct of warfare, environmental protection, international sovereignty, and human rights. Peacekeeping forces consisting of troops provided by various countries, with various United Nations and other aid agencies, helped to relieve famine, disease, and poverty, and to suppress some local armed conflicts. Europe slowly united, economically and, in some ways, politically, to form the European Union, which consisted of 15 European countries by the end of the 20th century.
Due to continuing industrialization and expanding trade, many significant changes of the century were, directly or indirectly, economic and technological in nature. Inventions such as the light bulb, the automobile, and the telephone in the late 19th century, followed by supertankers, airliners, motorways, radio, television, antibiotics, nuclear power, frozen food, computers and microcomputers, the Internet, and mobile telephones affected people's quality of life across the developed world. Scientific research, engineering professionalization and technological development—much of it motivated by the Cold War arms race—drove changes in everyday life.
At the beginning of the century, strong discrimination based on race and sex was significant in general society. Although the Atlantic slave trade had ended in the 19th century, the fight for equality for non-white people in the white-dominated societies of North America, Europe, and South Africa continued. During the century, the social taboo of sexism fell. By the end of the 20th century, women had the same legal rights as men in many parts of the world, and racism had come to be seen as abhorrent.  Attitudes towards homosexuality also began to change in the later part of the century.
Communications and information technology, transportation technology, and medical advances had radically altered daily lives. Europe appeared to be at a sustainable peace for the first time in recorded history. The people of the Indian subcontinent, a sixth of the world population at the end of the 20th century, had attained an indigenous independence for the first time in centuries. China, an ancient nation comprising a fifth of the world population, was finally open to the world, creating a new state after the near-complete destruction of the old cultural order. With the end of colonialism and the Cold War, nearly a billion people in Africa were left in new nation states after centuries of foreign domination.
The world was undergoing its second major period of globalization; the first, which started in the 18th century, having been terminated by World War I. Since the US was in a dominant position, a major part of the process was Americanization. The influence of China and India was also rising, as the world's largest populations were rapidly integrating with the world economy.
Terrorism, dictatorship, and the spread of nuclear weapons were pressing global issues. The world was still blighted by small-scale wars and other violent conflicts, fueled by competition over resources and by ethnic conflicts.
Disease threatened to destabilize many regions of the world. New viruses such as the West Nile virus continued to spread. Malaria and other diseases affected large populations. Millions were infected with HIV, the virus which causes AIDS. The virus was becoming an epidemic in southern Africa.
Based on research done by climate scientists, the majority of the scientific community consider that in the long term environmental problems may threaten the planet's habitability.  One argument is that of global warming occurring due to human-caused emission of greenhouse gases, particularly carbon dioxide produced by the burning of fossil fuels.  This prompted many nations to negotiate and sign the Kyoto treaty, which set mandatory limits on carbon dioxide emissions.
The number of people killed during the century by government actions was in the hundreds of millions. This includes deaths caused by wars, genocide, politicide and mass murders. The deaths from acts of war during the two world wars alone have been estimated at between 50 and 80 million.[ citation needed] Political scientist Rudolph Rummel estimated 262,000,000 deaths caused by democide, which excludes those killed in war battles, civilians unintentionally killed in war and killings of rioting mobs.  According to Charles Tilly, "Altogether, about 100 million people died as a direct result of action by organized military units backed by one government or another over the course of the century. Most likely a comparable number of civilians died of war-induced disease and other indirect effects."  It is estimated that approximately 70 million Europeans died through war, violence and famine between 1914 and 1945. 
- After gaining political rights in the United States and much of Europe in the first part of the century, and with the advent of new birth control techniques, women became more independent throughout the century.
- Rising nationalism and increasing national awareness were among the many causes of World War I (1914–1918), the first of two wars to involve many major world powers including Germany, France, Italy, Japan, Russia/ USSR, the British Empire and the United States. World War I led to the creation of many new countries, especially in Eastern Europe. At the time, it was said by many to be the " war to end all wars".
- Industrial warfare greatly increased in its scale and complexity during the first half of the 20th century. Notable developments included chemical warfare, the introduction of military aviation and the widespread use of submarines. The introduction of nuclear warfare in the mid-20th century marked the definite transition to modern warfare.
- Civil wars occurred in many nations. A violent civil war broke out in Spain in 1936 when General Francisco Franco rebelled against the Second Spanish Republic. Many  consider this war as a testing battleground for World War II, as the fascist armies bombed some Spanish territories.
- The Great Depression in the 1930s led to the rise of Fascism and Nazism in Europe.
- World War II (1939–1945) involved Eastern Asia and the Pacific, in the form of Japanese aggression against China and the United States. Civilians also suffered greatly in World War II, due to the aerial bombing of cities on both sides, and the German genocide of the Jews and others, known as the Holocaust.
- During World War I, in the Russian Revolution of 1917, 300 years of Romanov reign were ended and the Bolsheviks, under the leadership of Vladimir Lenin, established the world's first Communist state. After the Soviet Union's involvement in World War II, communism became a major force in global politics, notably in Eastern Europe, China, Indochina and Cuba, where communist parties gained near-absolute power.
- The Cold War (1947-1989) caused an arms race and increasing competition between the two major players in the world: the Soviet Union and the United States. This competition included the development and improvement of nuclear weapons and the Space Race. This led to the proxy wars with the Western bloc, including wars in Korea (1950–1953) and Vietnam (1957–1975).
- The Soviet authorities caused the deaths of millions of their own citizens in order to eliminate domestic opposition.  More than 18 million people passed through the Gulag, with a further 6 million being exiled to remote areas of the Soviet Union. 
- The civil rights movement in the United States and the movement against apartheid in South Africa challenged racial segregation in those countries.
- The two world wars led to efforts to increase international cooperation, notably through the founding of the League of Nations after World War I, and its successor, the United Nations, after World War II.
- Nationalist movements in the subcontinent led to the independence and partition of India and Pakistan.
- Gandhi's nonviolence and Indian independence movement against the British Empire influenced many political movements around the world, including the civil rights movement in the U.S., and freedom movements in South Africa and Burma.
- The creation of Israel in 1948, a Jewish state in the Middle East, at the end of the British Mandate for Palestine, fueled many regional conflicts. These were also influenced by the vast oil fields in many of the other countries of the predominantly Arab region.
- The end of colonialism led to the independence of many African and Asian countries. During the Cold War, many of these aligned with the United States, the USSR, or China for defense.
- After a long period of civil wars and conflicts with western powers, China's last imperial dynasty ended in 1912. The resulting republic was replaced, after another civil war, by a communist People's Republic in 1949. At the end of the 20th century, though still ruled by a communist party, China's economic system had largely transformed to capitalism.
- The Great Chinese Famine was a direct cause of the death of tens of millions of Chinese peasants between 1959 and 1962. It is thought to be the largest famine in human history. 
- The Vietnam War caused two million deaths, changed the dynamics between the Eastern and Western Blocs, and altered North-South relations. 
- The Soviet War in Afghanistan caused one million deaths and contributed to the downfall of the Soviet Union. 
- The revolutions of 1989 released Eastern and Central Europe from Soviet supremacy. Soon thereafter, the Soviet Union, Czechoslovakia, and Yugoslavia dissolved, the latter violently over several years, into successor states, many rife with ethnic nationalism. Meanwhile, East Germany and West Germany were reunified in 1990.
- The Tiananmen Square protests of 1989, culminating in the deaths of hundreds of civilian protesters, were a series of demonstrations in and near Tiananmen Square in Beijing, China. Led mainly by students and intellectuals, the protests occurred in a year that saw the collapse of a number of communist governments around the world.
- European integration began in earnest in the 1950s, and eventually led to the European Union, a political and economic union that comprised 15 countries at the end of the 20th century.
- As the century began, Paris was the artistic capital of the world, where both French and foreign writers, composers and visual artists gathered. By the end of the century New York City had become the artistic capital of the world.
- Theater, films, music and the media had a major influence on fashion and trends in all aspects of life. As many films and much music originate from the United States, American culture spread rapidly over the world.
- 1953 saw the coronation of Queen Elizabeth II, an iconic figure of the century.
- Visual culture became more dominant not only in films but in comics and television as well. During the century a new skilled understanding of narrativist imagery was developed.
- Computer games and internet surfing became new and popular form of entertainment during the last 25 years of the century.
- In literature, science fiction, fantasy (with well-developed fictional worlds, rich in detail), and alternative history fiction gained unprecedented popularity. Detective fiction gained unprecedented popularity in the interwar period. In the United States in 1961 Grove Press published Tropic of Cancer a novel by Henry Miller redefining pornography and censorship in publishing in America.
The invention of music recording technologies such as the phonograph record, and dissemination technologies such as radio broadcasting, massively expanded the audience for music. Prior to the 20th century, music was generally only experienced in live performances. Many new genres of music were established during the 20th century.
- Igor Stravinsky revolutionized classical composition.
- In the 1920s, Arnold Schoenberg developed the twelve-tone technique, which became widely influential on 20th-century composers.
- In classical music, composition branched out into many completely new domains, including dodecaphony, aleatoric (chance) music, and minimalism.
- Tango was created in Argentina and became extremely popular in the rest of the Americas and Europe.
- Blues and jazz music became popularized during the 1910s, 1920s and 1930s in the United States.
- Country music develops in the 1920s and 1930s in the United States.
- Blues and country went on to influence rock and roll in the 1950s, which along with folk music, increased in popularity with the British Invasion of the mid-to-late 1960s.
- Rock soon branched into many different genres, including folk rock, heavy metal, punk rock, and alternative rock and became the dominant genre of popular music.
- This was challenged with the rise of hip hop in the 1980s and 1990s.
- Other genres such as house, techno, reggae, and soul all developed during the latter half of the century and went through various periods of popularity.
- Synthesizers began to be employed widely in music and crossed over into the mainstream with new wave music in the 1980s. Electronic instruments have been widely deployed in all manners of popular music and has led to the development of such genres as house, synth-pop, electronic dance music, and industrial.
Film as an artistic medium was created in the 20th century. The first modern movie theatre was established in Pittsburgh in 1905.  Hollywood developed as the center of American film production. While the first films were in black and white, technicolor was developed in the 1920s to allow for color films. Sound films were developed, with the first full-length feature film, The Jazz Singer, released in 1927. The Academy Awards were established in 1929. Animation was also developed in the 1920s, with the first full-length cel animated feature film Snow White and the Seven Dwarfs, released in 1937. Computer-generated imagery was developed in the 1980s, with the first full-length CGI-animated film Toy Story was released in 1995.
- Julie Andrews, Harry Belafonte, Humphrey Bogart, Marlon Brando, James Cagney, Charlie Chaplin, Sean Connery, Tom Cruise, James Dean, Robert De Niro, Harrison Ford, Clark Gable, Cary Grant, Audrey Hepburn, Katharine Hepburn, Bruce Lee, Marilyn Monroe, Paul Newman, Jack Nicholson, Al Pacino, Sidney Poitier, Meryl Streep, Elizabeth Taylor, James Stewart, and John Wayne are among the most popular Hollywood stars of the 20th century.
- Oscar Micheaux, Sergei Eisenstein, D. W. Griffith, Cecil B. DeMille, Frank Capra, Howard Hawks, John Ford, Orson Welles, Martin Scorsese, John Huston, Alfred Hitchcock, Akira Kurosawa, Spike Lee, Ingmar Bergman, Federico Fellini, Walt Disney, Stanley Kubrick, Steven Spielberg, Ridley Scott, Woody Allen, Quentin Tarantino, James Cameron, William Friedkin and George Lucas are among the most important and popular filmmakers of the 20th century.
- In theater, sometimes referred to as Broadway in New York City, playwrights such as Eugene O'Neill, Samuel Beckett, Edward Albee, Arthur Miller and Tennessee Williams introduced innovative language and ideas to the idiom. In musical theater, figures such as Rodgers and Hammerstein, Lerner and Loewe, and Irving Berlin had an enormous impact on both film and the culture in general.
- Modern Dance is born in America as both a 'rebellion' against centuries-old European ballet, as well as born from the oppression in America. Dancers and choreographers Alvin Ailey, Isadora Duncan, Vaslav Nijinsky, Ruth St. Denis, Martha Graham, José Limón, Doris Humphrey, Merce Cunningham, and Paul Taylor re-defined movement, struggling to bring it back to its 'natural' roots and along with Jazz, created a solely American art form. Alvin Ailey is credited with popularizing modern dance and revolutionizing African-American participation in 20th-century concert dance. His company gained the nickname "Cultural Ambassador to the World" because of its extensive international touring. Ailey's choreographic masterpiece Revelations is believed to be the best known and most often seen modern dance performance.
- While already conceptualized in the 1940s– 50s, video games only emerged as an industry during the 1970s, and then exploded into social and cultural phenomenon such as the golden age of arcade video games, with notable releases such as Taito's Space Invaders, Atari's Asteroids, and Namco's Pac-Man,  the worldwide success of Nintendo's Super Mario Bros.  and the release in the 1990s of Sony PlayStation console, the first one to break the record of 100 million units sold. 
- Video game design becomes a discipline and a job. Some game designers in this century stand out for their work, as Shigeru Miyamoto, Hideo Kojima, Sid Meier and Will Wright.
- The art world experienced the development of new styles and explorations such as fauvism, expressionism, Dadaism, cubism, de stijl, surrealism, abstract expressionism, color field, pop art, minimal art, lyrical abstraction, and conceptual art.
- The modern art movement revolutionized art and culture and set the stage for both Modernism and its counterpart postmodern art as well as other contemporary art practices.
- Art Nouveau began as advanced architecture and design but fell out of fashion after World War I. The style was dynamic and inventive but unsuited to the depression of the Great War.
- In Europe, modern architecture departed from the decorated styles of the Victorian era. Streamlined forms inspired by machines became commonplace, enabled by developments in building materials and technologies. Before World War II, many European architects moved to the United States, where modern architecture continued to develop.
- The automobile increased the mobility of people in the Western countries in the early-to-mid-century, and in many other places by the end of the 20th century. City design throughout most of the West became focused on transport via car.
- The popularity of sport increased considerably—both as an activity for all, and as entertainment, particularly on television.
- The modern Olympic Games, first held in 1896, grew to include tens of thousands of athletes in dozens of sports.
- The FIFA World Cup was first held in 1930, and was held every four years after World War II.
Multiple new fields of mathematics were developed in the 20th century. In the first part of the 20th century, measure theory, functional analysis, and topology were established, and significant developments were made in fields such as abstract algebra and probability. The development of set theory and formal logic led to Gödel's incompleteness theorems.
Later in the 20th century, the development of computers led to the establishment of a theory of computation.  Other computationally-intense results include the study of fractals  and a proof of the four color theorem in 1976. 
- New areas of physics, like special relativity, general relativity, and quantum mechanics, were developed during the first half of the century. In the process, the internal structure of atoms came to be clearly understood, followed by the discovery of elementary particles.
- It was found that all the known forces can be traced to only four fundamental interactions. It was discovered further that two forces, electromagnetism and weak interaction, can be merged in the electroweak interaction, leaving only three different fundamental interactions.
- Discovery of nuclear reactions, in particular nuclear fusion, finally revealed the source of solar energy.
- Radiocarbon dating was invented, and became a powerful technique for determining the age of prehistoric animals and plants as well as historical objects.
- A much better understanding of the evolution of the universe was achieved, its age (about 13.8 billion years) was determined, and the Big Bang theory on its origin was proposed and generally accepted.
- The age of the Solar System, including Earth, was determined, and it turned out to be much older than believed earlier: more than 4 billion years, rather than the 20 million years suggested by Lord Kelvin in 1862. 
- The planets of the Solar System and their moons were closely observed via numerous space probes. Pluto was discovered in 1930 on the edge of the solar system, although in the early 21st century, it was reclassified as a dwarf planet instead of a planet proper, leaving eight planets.
- No trace of life was discovered on any of the other planets in the Solar System (or elsewhere in the universe), although it remained undetermined whether some forms of primitive life might exist, or might have existed, somewhere. Extrasolar planets were observed for the first time.
- Genetics was unanimously accepted and significantly developed. The structure of DNA was determined in 1953 by James Watson,   Francis Crick,   Rosalind Franklin  and Maurice Wilkins,   following by developing techniques which allow to read DNA sequences and culminating in starting the Human Genome Project (not finished in the 20th century) and cloning the first mammal in 1996.
- The role of sexual reproduction in evolution was understood, and bacterial conjugation was discovered.
- The convergence of various sciences for the formulation of the modern evolutionary synthesis (produced between 1936 and 1947), providing a widely accepted account of evolution.
- Placebo- controlled, randomized, blinded clinical trials became a powerful tool for testing new medicines.
- Antibiotics drastically reduced mortality from bacterial diseases and their prevalence.
- A vaccine was developed for polio, ending a worldwide epidemic. Effective vaccines were also developed for a number of other serious infectious diseases, including influenza, diphtheria, pertussis (whooping cough), tetanus, measles, mumps, rubella (German measles), chickenpox, hepatitis A, and hepatitis B.
- Epidemiology and vaccination led to the eradication of the smallpox virus in humans.
- X-rays became powerful diagnostic tool for wide spectrum of diseases, from bone fractures to cancer. In the 1960s, computerized tomography was invented. Other important diagnostic tools developed were sonography and magnetic resonance imaging.
- Development of vitamins virtually eliminated scurvy and other vitamin-deficiency diseases from industrialized societies.
- New psychiatric drugs were developed. These include antipsychotics for treating hallucinations and delusions, and antidepressants for treating depression.
- The role of tobacco smoking in the causation of cancer and other diseases was proven during the 1950s (see British Doctors Study).
- New methods for cancer treatment, including chemotherapy, radiation therapy, and immunotherapy, were developed. As a result, cancer could often be cured or placed in remission.
- The development of blood typing and blood banking made blood transfusion safe and widely available.
- The invention and development of immunosuppressive drugs and tissue typing made organ and tissue transplantation a clinical reality.
- New methods for heart surgery were developed, including pacemakers and artificial hearts.
- Cocaine/ crack and heroin were found to be dangerous addictive drugs, and their wide usage had been outlawed; mind-altering drugs such as LSD and MDMA were discovered and later outlawed. In many countries, a war on drugs caused prices to soar 10–20 times higher, leading to profitable black market drugdealing, and to prison inmate sentences being 80% related to drug use by the 1990s.[ citation needed]
- Contraceptive drugs were developed, which reduced population growth rates in industrialized countries, as well as decreased the taboo of premarital sex throughout many western countries.
- The development of medical insulin during the 1920s helped raise the life expectancy of diabetics to three times of what it had been earlier.
- Vaccines, hygiene and clean water improved health and decreased mortality rates, especially among infants and the young.
- An influenza pandemic, Spanish Flu, killed anywhere from 17 to 100 million people between 1918 and 1919.
- A new viral disease, called the Human Immunodeficiency Virus, or HIV, arose in Africa and subsequently killed millions of people throughout the world. HIV leads to a syndrome called Acquired Immunodeficiency Syndrome, or AIDS. Treatments for HIV remained inaccessible to many people living with AIDS and HIV in developing countries, and a cure has yet to be discovered.
- Because of increased life spans, the prevalence of cancer, Alzheimer's disease, Parkinson's disease, and other diseases of old age increased slightly.
- Sedentary lifestyles, due to labor-saving devices and technology, along with the increase in home entertainment and technology such as television, video games, and the internet contributed to an "epidemic" of obesity, at first in the rich countries, but by the end of the 20th century spreading to the developing world.
- The dominant use of fossil sources and nuclear power, considered the conventional energy sources.
- Widespread use of petroleum in industry—both as a chemical precursor to plastics and as a fuel for the automobile and airplane—led to the geopolitical importance of petroleum resources. The Middle East, home to many of the world's oil deposits, became a center of geopolitical and military tension throughout the latter half of the century. (For example, oil was a factor in Japan's decision to go to war against the United States in 1941, and the oil cartel, OPEC, used an oil embargo of sorts in the wake of the Yom Kippur War in the 1970s).
- The increase in fossil fuel consumption also fueled a major scientific controversy over its effect on air pollution, global warming, and global climate change.
- Pesticides, herbicides and other toxic chemicals accumulated in the environment, including in the bodies of humans and other animals.
- Population growth and worldwide deforestation diminished the quality of the environment.
- In the last third of the century, concern about humankind's impact on the Earth's environment made environmentalism popular. In many countries, especially in Europe, the movement was channeled into politics through Green parties. Increasing awareness of global warming began in the 1980s, commencing decades of social and political debate.
One of the prominent traits of the 20th century was the dramatic growth of technology. Organized research and practice of science led to advancement in the fields of communication, electronics, engineering, travel, medicine, and war.
- Basic home appliances including washing machines, clothes dryers, furnaces, exercise machines, refrigerators, freezers, electric stoves and vacuum cleaners became popular from the 1920s through the 1950s. Radios were popularized as a form of entertainment during the 1920s, which extended to television during the 1950s.
- The first airplane, the Wright Flyer, was flown in 1903. With the engineering of the faster jet engine in the 1940s, mass air travel became commercially viable.
- The assembly line made mass production of the automobile viable. By the end of the 20th century, billions of people had automobiles for personal transportation. The combination of the automobile, motor boats and air travel allowed for unprecedented personal mobility. In western nations, motor vehicle accidents became the greatest cause of death for young people. However, expansion of divided highways reduced the death rate.
- The triode tube was invented.
- New materials, most notably stainless steel, Velcro, silicone, teflon, and plastics such as polystyrene, PVC, polyethylene, and nylon came into widespread use for many various applications. These materials typically have tremendous performance gains in strength, temperature, chemical resistance, or mechanical properties over those known prior to the 20th century.
- Aluminum became an inexpensive metal and became second only to iron in use.
- Thousands of chemicals were developed for industrial processing and home use.
- The Space Race between the United States and the Soviet Union gave a peaceful outlet to the political and military tensions of the Cold War, leading to the first human spaceflight with the Soviet Union's Vostok 1 mission in 1961, and man's first landing on another world—the Moon—with America's Apollo 11 mission in 1969. Later, the first space station was launched by the Soviet space program. The United States developed the first reusable spacecraft system with the Space Shuttle program, first launched in 1981. As the century ended, a permanent manned presence in space was being founded with the ongoing construction of the International Space Station.
- In addition to human spaceflight, unmanned space probes became a practical and relatively inexpensive form of exploration. The first orbiting space probe, Sputnik 1, was launched by the Soviet Union in 1957. Over time, a massive system of artificial satellites was placed into orbit around Earth. These satellites greatly advanced navigation, communications, military intelligence, geology, climate, and numerous other fields. Also, by the end of the 20th century, unmanned probes had visited the Moon, Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune, and various asteroids and comets. The Hubble Space Telescope, launched in 1990, greatly expanded our understanding of the Universe and brought brilliant images to TV and computer screens around the world.
- The Global Positioning System, a series of satellites that allow land-based receivers to determine their exact location, was developed and deployed. 
A technological revolution began in the late 20th century, variously called the Digital Revolution, the information revolution,    the electronics revolution,  the microelectronic revolution,  the Information Age,  the silicon revolution,   the Silicon Age,    and/or the third industrial revolution. 
- The first transistor was invented by John Bardeen and Walter Houser Brattain at Bell Labs in 1947.  However, early junction transistors were relatively bulky devices that were difficult to manufacture on a mass-production basis, which limited them to a number of specialised applications. 
- The MOSFET (metal-oxide-silicon field-effect transistor), also known as the MOS transistor, was invented by Mohamed M. Atalla and Dawon Kahng at Bell Labs,     in November 1959.  It was the first truly compact transistor that could be miniaturised and mass-produced for a wide range of uses.  The widespread adoption of MOSFETs revolutionized the electronics industry,  becoming the fundamental building block of the Digital Revolution   and "the base technology" of the late 20th to early 21st centuries.  The MOSFET went on to become the most widely manufactured device in history.  
- Semiconductor materials were discovered, and methods of production and purification developed for use in electronic devices. Silicon became one of the purest substances ever produced. The wide adoption of the MOSFET led to silicon becoming the dominant manufacturing material during the late 20th century to early 21st century,   a period that has been called the Silicon Age,    similar to how the Stone Age, Bronze Age and Iron Age were defined by the dominant materials during their respective ages of civilization. 
- The MOS integrated circuit chip (a silicon integrated circuit chip built from MOSFETs) revolutionized electronics and computers. The MOS chip was invented in the early 1960s.  The silicon-gate MOS chip later developed by Federico Faggin in 1968 was the basis for the first single-chip microprocessor, the Intel 4004, in 1971.  MOS integrated circuits and microprocessors led to the microcomputer revolution,  the proliferation of the personal computer in the 1980s, and then cell phones and the public-use Internet in the 1990s.
- Metal-oxide-semiconductor (MOS) image sensors, which first began appearing in the late 1960s, led to the transition from analog to digital imaging, and from analog to digital cameras, during the 1980s–1990s.  
- Discrete cosine transform (DCT) coding, a data compression technique first proposed in 1972,  enabled practical digital media transmission in the 1990s,    with image compression formats such as JPEG (1992), video coding formats such as H.26x (1988 onwards) and MPEG (1993 onwards),  audio coding standards such as Dolby Digital (1991)   and MP3 (1994),  and digital television standards such as video-on-demand (VOD)  and high-definition television (HDTV). 
- The number and types of home appliances increased dramatically due to advancements in technology, the wide adoption of MOSFETs, electricity availability, the transition from analog to digital media, and increases in wealth and leisure time. The microwave oven became popular during the 1980s and have become a standard in all homes by the 1990s. Cable and satellite television spread rapidly during the 1980s and 1990s. Personal computers began to enter the home during the 1970s–1980s as well.
- Video games were popularized during the late 1970s to 1980s, with the golden age of arcade video games.
- The age of the portable music player was enabled by the development of the transistor radio, 8-track and cassette tapes in the 1960s, which slowly began to replace record players, culminating in the Sony Walkman in the late 1970s. These were in turn replaced by the digital compact disc (CD) during the 1980s to 1990s. The proliferation of the MDCT-based MP3 audio coding format  on the Internet during the mid-to-late 1990s made digital distribution of music possible.
- Video cassette recorders (VCRs) were popularized in the 1970s, but by the end of the 20th century, DVD players were beginning to replace them, making the VHS obsolete by the end of the first decade of the 21st century.
- There was a rapid growth of the telecommunications industry towards the end of the 20th century, driven by the development of large-scale integration (LSI) complementary MOS (CMOS) technology, information theory, digital signal processing, and wireless communications such as cellular networks and mobile telephony. 
- The Vatican II council was held from 1962 to 1965, and resulted in significant changes in the Catholic Church.
- The Wahhabi sect of Sunni Islam gained in influence with the growth of Saudi Arabia.
- Multiple new religions were founded, including the Nation of Islam, Scientology, and the Pentecostalism movement.
- Atheism became considerably more common, both in secular Western countries, and Communist countries with a policy of state atheism.
- The Great Depression was a worldwide economic slowdown that lasted throughout the early 1930s.
- The Soviet Union implemented a series of five-year plans for industrialization and economic development.
- Most countries abandoned the gold standard for their currency. The Bretton Woods system involved currencies being pegged to the United States dollar; after the system collapsed in 1971 most major currencies had a floating exchange rate.
- 20th-century inventions
- Death rates in the 20th century
- Infectious disease in the 20th century
- Modern art
- Short twentieth century
- Timelines of modern history
- List of 20th-century women artists
- List of notable 20th-century writers
- List of 20th-century American writers by birth year
- List of battles 1901–2000
- List of stories set in a future now past
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