The British Empire comprised the dominions, colonies, protectorates, mandates and other territories ruled or administered by the United Kingdom. It originated with the overseas possessions and trading posts established by England between the late 16th and early 18th centuries. At its height, it was the largest empire in history and, for over a century, was the foremost global power. By 1922 the British Empire held sway over about 458 million people, one-fifth of the world's population at the time. The empire covered more than 33,700,000 km² (13,012,000 sq mi), almost a quarter of the Earth's total land area. As a result, its political, legal, linguistic and cultural legacy is widespread. At the peak of its power, the phrase "the empire on which the sun never sets" was often used to describe the British Empire, because its expanse across the globe meant that 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 globe, and in the process established large overseas empires. Envious of the great wealth these empires generated, England, France, and the Netherlands, began to establish colonies and trade networks of their own in the Americas and Asia. A series of wars in the 17th and 18th centuries with the Netherlands and France left England (and then, following union between England and Scotland in 1707, Great Britain) the dominant colonial power in North America and India.

The independence of the Thirteen Colonies in North America in 1783 after the American Revolutionary War caused Britain to lose of some of its oldest and most populous colonies. British attention soon turned towards Asia, Africa, and the Pacific. Following the defeat of Napoleonic France in 1815, Britain enjoyed a century of almost unchallenged dominance and expanded its imperial holdings across the globe. Increasing degrees of autonomy were granted to its white settler colonies, some of which were reclassified as dominions.