At the zenith of the British Empire, Great Britain ruled lands on every continent and islands in every ocean. It was a common saying that the sun never set on Britain's dominions. As one after another of these lands have become independent states, they have joined together in the Commonwealth (from 1931 until 1949, the British Commonwealth of Nations). The territory of its member states covers almost a quarter of the land surface of the Earth and contains nearly a fourth of its people. It comprises peoples of every race and many religions and includes some of the oldest as well as some of the youngest civilizations.

The Commonwealth is a free, voluntary association of sovereign states, together with dependencies for which certain states are responsible. The sovereign member states are free and equal partners with Great Britain in the association. When a dependency achieves full sovereignty, it may decide whether or not to be a member. A member may elect to leave the Commonwealth but few states have left.