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 became 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 percent 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. At the peak of its power, 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 of their own 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 War of Independence 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. Increasing degrees of autonomy were granted to 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.

The foundations of the British Empire were laid when England and Scotland were separate kingdoms. In 1496, King Henry VII of England, following the successes of Spain and Portugal in overseas exploration, commissioned John Cabot to lead an expedition to discover a northwest passage to Asia via the North Atlantic. Cabot sailed in 1497, five years after the first voyage of Christopher Columbus, and made landfall on the coast of Newfoundland. He believed he had reached Asia, and there was no attempt to found a colony. Cabot led another voyage to the Americas the following year but did not return; it is unknown what happened to his ships. There is "inferential evidence [that] suggests that Bristol mariners may already have known of that North American shore and had been harvesting its rich seas for a decade or more", but England made no formal claims.

No further attempts to establish English colonies in the Americas were made until well into the reign of Queen Elizabeth I, during the last decades of the 16th century. In the meantime, Henry VIII's 1533 Statute in Restraint of Appeals had declared "that this realm of England is an Empire". The Protestant Reformation turned England and Catholic Spain into implacable enemies. In 1562, Elizabeth I encouraged the privateers John Hawkins and Francis Drake to engage in slave-raiding attacks against Spanish and Portuguese ships off the coast of West Africa with the aim of establishing an Atlantic slave trade. This effort was rebuffed and later, as the Anglo-Spanish Wars intensified, Elizabeth I gave her blessing to further privateering raids against Spanish ports in the Americas and shipping that was returning across the Atlantic, laden with treasure from the New World. At the same time, influential writers such as Richard Hakluyt and John Dee (who was the first to use the term "British Empire") were beginning to press for the establishment of England's own empire. By this time, Spain had become the dominant power in the Americas and was exploring the Pacific Ocean, Portugal had established trading posts and forts from the coasts of Africa and Brazil to China, and France had begun to settle the Saint Lawrence River area, later to become New France.

Although England tended to trail behind Portugal, Spain, and France in establishing overseas colonies, it carried out its first modern colonisation, referred to as the Munster Plantations, in 16th century Ireland by settling it with English and Welsh Protestant settlers. England had already colonised part of the country following the Norman invasion of Ireland in 1169. Several people who helped establish the Munster plantations later played a part in the early colonisation of North America, particularly a group known as the West Country Men.

In 1578, Elizabeth I granted a patent to Humphrey Gilbert for discovery and overseas exploration. That year, Gilbert sailed for the Caribbean with the intention of engaging in piracy and establishing a colony in North America, but the expedition was aborted before it had crossed the Atlantic. In 1583, he embarked on a second attempt. On this occasion, he formally claimed the harbour of the island of Newfoundland, although no settlers were left behind. Gilbert did not survive the return journey to England and was succeeded by his half-brother, Walter Raleigh, who was granted his own patent by Elizabeth in 1584. Later that year, Raleigh founded the Roanoke Colony on the coast of present-day North Carolina, but lack of supplies caused the colony to fail.