A member state of the European Union is a state that is party to treaties of the European Union (EU) and thereby subject to the privileges and obligations of EU membership. Unlike the membership of an international organisation, EU membership places each member under binding laws in exchange for representation in the EU's legislative and judicial institutions. On the other hand, EU states retain considerable autonomy compared to the constituent states of a federation (such as a U.S. state), maintaining their national military and foreign policy (where they have not agreed to European action in these areas).

There are twenty-seven member states of the EU. Six core states founded the EU's predecessor, the European Economic Community, in 1957 and the remaining states joined in subsequent enlargements. Before being allowed to join the EU, a state must fulfil the economic and political conditions generally known as the Copenhagen criteria. These essentially require a candidate to have a democratic, free market government together with the corresponding freedoms and institutions, and respect the rule of law. Enlargement of the Union is contingent upon the agreement of each existing member and the candidate's adoption of all pre-existing EU law.