A Member State of the European Union is any one of the 27 sovereign states that have acceded to the European Union (EU) since its de facto inception in 1951 as the European Coal and Steel Community (ECSC). From an original membership of six states, there have been six successive enlargements, the largest occurring on 1 May 2004, when ten states joined. The EU is currently composed of twenty republics, six kingdoms, and one grand duchy.

Bulgaria and Romania are the most recent Member States, joining on 1 January 2007. Negotiations are also under way with a number of other states. The process of enlargement is sometimes referred to as European integration. However, this term is also used to refer to the intensification of cooperation between EU Member States as national governments allow for the gradual harmonisation of national laws. Before being allowed to join the European Union, a state must fulfil the economic and political conditions generally known as the Copenhagen criteria. These basically require that a candidate Member State must enjoy a secular, democratic system of government, together with the corresponding freedoms and institutions, and respect the rule of law. Under the terms of the Treaty on European Union, enlargement of the Union is conditional upon the agreement of each existing Member State as well as approval by the European Parliament.