The countries of the United Kingdom are England, Northern Ireland, Scotland and Wales: four countries that together form the sovereign state of the United Kingdom. Prior to 1922, the entire island of Ireland rather than just Northern Ireland was one of the countries. The alternative term Home Nations is also used, although today this is mainly in sporting contexts and may still include all of Ireland.

The United Kingdom, a sovereign state under international law, is a member of intergovernmental organisations, the European Union and the United Nations. England, Northern Ireland, Scotland and Wales are not themselves listed in the International Organization for Standardization (ISO) list of countries. However the ISO list of the subdivisions of the UK is supplied by British Standards and the Office for National Statistics and so uses "country" to describe England, Scotland and Wales. Northern Ireland, in contrast, is described as a "province" in the same lists. The Parliament of the United Kingdom and Government of the United Kingdom deal with all reserved matters for Northern Ireland and Scotland and all non-transferred matters for Wales, but not in general matters that have been devolved to the Northern Ireland Assembly, Scottish Parliament and National Assembly for Wales. Additionally, devolution in Northern Ireland is conditional on co-operation between the Northern Ireland Executive and the Government of Ireland (see North/South Ministerial Council). The Government of the United Kingdom also consults with the Government of Ireland to reach agreement on some non-devolved matters for Northern Ireland (see British-Irish Intergovernmental Conference). England remains the full responsibility of the Parliament of the United Kingdom, which is centralised in London.