20th century

The Twentieth Century of the Common Era began on January 1, 1901 and ended on December 31, 2000, according to the Gregorian calendar, (2000 was the first century leap year since 1600)

The Earth seen from Apollo 17. The second half of the 20th century saw an increase of interest in space exploration.

The Russian Empire, the German Empire, the Ottoman Empire, and the Austro-Hungarian Empire dissolved in the first half of the century, with all but the British Empire collapsing during the course of World War I. The inter-war years saw the Great Depression cause a massive disruption to the world economy. Shortly afterwards, World War II broke out, pitting the Allied powers (chiefly the Soviet Union, the United Kingdom, and the United States) against the Axis powers (chiefly Nazi Germany, the Empire of Japan, and Italy) which eventually resulted in a total victory for the Allies, but at the cost of over 60 million lives and complete devastation of countless nations. As means to prevent future world wars, the United Nations was formed, however disagreements between the two new superpowers, the Soviet Union and the United States, resulted in the Cold War, which dominated geopolitical life for 40 years. The Soviet Union broke up in 1991, resulting in the United States taking on sole superpower status, although by the end of the century China, India, and the European Union had greatly increased their influence.

The century saw a major shift in the way that vast numbers of people lived, as a result of political, ideological, economic, social, cultural, scientific, technological, and medical change and progress. Terms like ideology, world war, genocide, and nuclear war entered common usage. Scientific discoveries, such as the theory of relativity and quantum physics, drastically changed the worldview of scientists, causing them to realize that the universe was fantastically more complex than previously believed, and dashing the hopes at the end of the 19th century that the last few details of scientific knowledge were about to be filled in. Accelerating scientific understanding, more efficient communications, and faster transportation transformed the world in those hundred years more rapidly and widely than at any time in the past. It was a century that started with horses, simple automobiles, and freighters but ended with luxury sedans, cruise ships, airliners and the space shuttle. Western society's basic form of personal transportation for thousands of years, was replaced by automobiles and buses within the span of a few decades. These developments were made possible by the large-scale exploitation of fossil fuel resources (especially petroleum), which offered large amounts of energy in an easily portable form, but also caused widespread concerns about pollution and long-term impact on the environment. Humanity explored outer space for the first time, even taking their first footsteps on the Moon.

Mass media, telecommunications, and information technology (especially computers, paperback books, public education, and the Internet) put the world's knowledge more widely available to people. Many people's view of the world changed significantly as they became much more aware of the struggles of others and, as such, became increasingly concerned with human rights. Advancements in medical technology also improved the welfare of many people: the life expectancy of the world increased from 35 years to 65 years. Rapid technological advancements, however, also allowed warfare to reach an unprecedented high: World War II alone killed over 60 million people, while nuclear weapons gave humankind the means to destroy itself in a very short period of time. 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 true global economy by the end of the century.