Separated from the European continent by the North Sea and English Channel, the United Kingdom (informally referred to as Britain) includes England, Scotland, Wales, and Northern Ireland. England and Wales were united in 1536. The addition of Scotland in 1707 created Great Britain, renamed the United Kingdom in 1801 when Ireland was added. The Republic of Ireland fought itself free of British rule in 1922, leaving volatile Northern Ireland as a province of the United Kingdom. About 55 percent of Northern Ireland's 1.6 million people trace their ancestry to Scotland or England, are Protestants, and favor continued union with Britain; however, many of the Roman Catholic population (44 percent) want to join the Republic of Ireland.

England is the most populous part of the U.K., with 49 million inhabitants. Almost one third of England's people live in the prosperous southeastern part of the country centered on London—one of the largest cities in Europe. Scotland, with one third of Britain's area, is a mountainous land with 5 million people, most of them (75 percent) concentrated in the lowland area where Glasgow and Edinburgh (Scotland's capital) are located. The Scottish nation can be traced to the Scoti, a Gaelic-speaking Celtic tribe. Wales, with 2.9 million people, is also mountainous with a Celtic culture—the country is called Cymru (pronounced CUM-ree) in the Welsh language—and its capital, Cardiff, features castles and museums highlighting Welsh culture. Since 1997 the government has been pursuing a policy of devolution, leading in 1999 to an elected Scottish parliament and Welsh assembly. In 2000 Londoners elected their first mayor and assembly.

The industrial revolution was born in Britain in the 18th century, making it the world's first industrialized nation. The British Empire, a worldwide system of dependencies, fed raw materials to British industry and spread British culture. Most dependencies gained independence in the 20th century. Part of the legacy of empire is that Britain is home to a growing multicultural population. The 2001 census counted more than 2.5 million Asians (mostly Indians and Pakistanis) and 1.1 million Blacks (from Africa and the Caribbean). Most of the remaining dependencies consist of small islands in the Atlantic and Caribbean.