Decolonization is the undoing of colonialism, the latter being the process whereby imperial nations establish and dominate foreign territories, often overseas. The meanings and applications of the term are disputed. Some scholars of decolonization focus especially on independence movements in the colonies and the collapse of global colonial empires.

As a movement to establish independence for colonized territories from their respective metropoles, decolonization began in 1775 with the American Revolution in North America against the British Empire. The Napoleonic Wars in the 19th century saw the French colonial empire, the Spanish Empire, and Portugal face decolonization with the Haitian Revolution, the Spanish American wars of independence, and the independence of Brazil from Portugal. A major wave of decolonization occurred in the aftermath of the First World War, including in the United States and the Empire of Japan. Another wave of decolonization occurred after the Second World War, and many countries gained their independence in the following years. The last wave of decolonization occurred after the Cold War with the collapse of the Soviet Union, the independence of Palau, and the handovers of Hong Kong and Macau. Seventeen territories remain under the United Nations classification of non-self-governing territories.

According to David Strang, decolonization is achieved through the attainment of sovereign statehood with de jure recognition by the international community or through full incorporation into an existing sovereign state.

The United Nations (UN) states that the fundamental right to self-determination is the core requirement for decolonization, and that this right can be exercised with or without political independence. A UN General Assembly Resolution in 1960 characterised colonial foreign rule as a violation of human rights. In states that have won independence, Indigenous people living under settler colonialism continue to make demands for decolonization and self-determination.

Although discussions of hegemony and power, central to the concept of decolonization, can be found as early as the writings of Thucydides, there have been several particularly active periods of decolonization in modern times. These include the decolonization of Africa, the breakup of the Spanish Empire in the 19th century; of the German, Austro-Hungarian, Ottoman, and Russian Empires following World War I; of the British, French, Dutch, Portuguese, Belgian, Italian, and Japanese Empires following World War II; and of the Soviet Union at the end of the Cold War.

Early studies of decolonisation appeared in the 1960s and 1970s. An important book from this period was The Wretched of the Earth (1961) by Martiniquan author Frantz Fanon, which established many aspects of decolonisation that would be considered in later works. Subsequent studies of decolonisation addressed economic disparities as a legacy of colonialism as well as the annihilation of people's cultures. Ngũgĩ wa Thiong'o explored the cultural and linguistic legacies of colonialism in the influential book Decolonising the Mind (1986).

"Decolonization" has also been used to refer to the intellectual decolonization from the colonizers' ideas that made the colonized feel inferior. Issues of decolonization persist and are raised contemporarily. In the Americas and South Africa, such issues are increasingly discussed under the term decoloniality.

In the two hundred years following the American Revolutionary War in 1783, 165 colonies have gained independence from Western imperial powers. Several analyses point to different reasons for the spread of anti-colonial political movements. Institutional arguments suggest that increasing levels of education in the colonies led to calls for popular sovereignty; Marxist analyses view decolonization as a result of economic shifts toward wage labor and an enlarged bourgeois class; yet another argument sees decolonization as a diffusion process wherein earlier revolutionary movements inspired later ones. Other explanations emphasize how the lower profitability of colonization and the costs associated with empire prompted decolonization. Some explanations emphasize how colonial powers struggled militarily against insurgents in the colonies due to a shift from 19th century conditions of "strong political will, a permissive international environment, access to local collaborators, and flexibility to pick their battles" to 20th century conditions of "apathetic publics, hostile superpowers, vanishing collaborators, and constrained options". In other words, colonial powers had more support from their own region in pursuing colonies in the 19th century than they did in the 20th century, where holding on to such colonies was often understood to be a burden.