A European Union member state is any one of the twenty-seven countries that have joined the European Union (EU) since its inception in 1958 as the European Economic Community (EEC). From an original membership of six states, there have been six successive enlargements, the largest occurring on 1 May 2004, when ten member states joined. Following the addition of Romania and Bulgaria in 2007, the EU's membership now stands at twenty-seven. 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 centralising of power within European institutions. 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.

Bulgaria and Romania comprise the second part of the EU's fifth enlargement, and joined the EU on 1 January 2007. This date was agreed upon at the Thessaloniki Summit in 2003, confirmed at Brussels on 18 June 2004, and affirmed by the country reports of October 2004 and the final report delivered on 26 September 2006. Bulgaria and Romania signed their Treaty of Accession on 25 April 2005 at a ceremony held at Neumünster Abbey in Luxembourg.