The Commonwealth of Nations, often referred to as the British Commonwealth or simply the Commonwealth, is an international association of 56 member states, the vast majority of which are former territories of the British Empire from which it developed. They are connected through their use of the English language and cultural and historical ties. The chief institutions of the association are the Commonwealth Secretariat, which focuses on intergovernmental relations, and the Commonwealth Foundation, which focuses on non-governmental relations between member nations. Numerous organisations are associated with and operate within the Commonwealth.

The Commonwealth dates back to the first half of the 20th century with the decolonisation of the British Empire through increased self-governance of its territories. It was created as the British Commonwealth of Nations through the Balfour Declaration at the 1926 Imperial Conference, and formalised by the United Kingdom through the Statute of Westminster in 1931. In 1949, the London Declaration allowed India to remain in the Commonwealth as a republic, marking a significant evolution of the association.

The Head of the Commonwealth is Charles III. He is king of 15 member states, known as the Commonwealth realms, while 36 other members are republics, and five others have different monarchs. Although he became head upon the death of his mother, Elizabeth II, the position is not technically hereditary.

Commonwealth citizens enjoy benefits in some member countries, particularly in the United Kingdom, and Commonwealth countries are represented to one another by high commissions rather than embassies. Member states have no legal obligations to one another, though various economic, judicial and military arrangements exist between countries. The Commonwealth Charter defines their shared values of democracy, human rights and the rule of law, as promoted by the quadrennial Commonwealth Games.

A majority of Commonwealth countries are small states, with small island developing states constituting almost half its membership.

Queen Elizabeth II, in her address to Canada on Dominion Day in 1959, pointed out that the Confederation of Canada on 1 July 1867 had been the birth of the "first independent country within the British Empire". She declared: "So, it also marks the beginning of that free association of independent states which is now known as the Commonwealth of Nations." As long ago as 18 January 1884 Lord Rosebery, while visiting Adelaide, South Australia, had described the changing British Empire, as some of its colonies became more independent, as a "Commonwealth of Nations". Conferences of British and colonial prime ministers occurred periodically from the first one in 1887, leading to the creation of the Imperial Conferences in 1911.

The Commonwealth developed from the imperial conferences. A specific proposal was presented by Jan Smuts in 1917 when he coined the term "the British Commonwealth of Nations" and envisioned the "future constitutional relations and readjustments in essence" at the Paris Peace Conference of 1919, attended by delegates from the Dominions as well as the United Kingdom. The term first received imperial statutory recognition in the Anglo-Irish Treaty of 1921, when the term British Commonwealth of Nations was substituted for British Empire in the wording of the oath taken by members of parliament of the Irish Free State.

In the Balfour Declaration at the 1926 Imperial Conference, the United Kingdom and its dominions agreed that they were "autonomous Communities within the British Empire, equal in status, in no way subordinate one to another in any aspect of their domestic or external affairs, though united by a common allegiance to the Crown, and freely associated as members of the British Commonwealth of Nations." The term 'Commonwealth' was officially adopted to describe the community.