The European Union (EU) was created by six founding states in 1957 (following the earlier establishment by the same six states of the European Coal and Steel Community in 1952) and has grown to 27 member states. There have been five enlargements, with the largest occurring on May 1, 2004, when 10 new member states joined, and the most recent on January 1, 2007, when Bulgaria and Romania joined.

Currently, accession negotiations are underway with several 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.

In order to join the European Union, a state needs to fulfill the economic and political conditions generally known as the Copenhagen criteria (after the Copenhagen summit in June 1993). That requires a secular, democratic government, rule of law and corresponding freedoms and institutions. According to the EU Treaty, each current member state and also the European Parliament have to agree to any enlargement.

The present EU Treaty - the Treaty of Nice - does not provide for the voting arrangements to be adopted for more than the present 27 members. Although the proposed European Constitution did provide such a mechanism, the ratification of this Treaty is currently on hold. The newly signed Treaty of Lisbon provides this mechanism, but has yet to be ratified.

Enlargement may lead to negative commercial consequences for other WTO Member States. The EU and the US have agreed in March 2006 to provide compensation for negative consequences associated with the 2004 enlargement.