On 1 May 2004, the European Union was enlarged to include Cyprus, Estonia, Hungary, Latvia, Lithuania, Malta, Poland, Slovakia, Slovenia and the Czech Republic.

To join the European Union, a country must fulfil economic and political conditions known as the ‘Copenhagen criteria’ (1993), according to which a prospective member must:
- be a stable democracy, respecting the rule of law, human rights and the protection of minorities;
- have a functioning market economy;
- adopt the common rules, standards and policies, the acquis communautaire, that make up the body of EU law.

Upon receiving an application for EU membership, the Council requests an opinion (avis) from the Commission, which presents a detailed description of the political and economic situation in the applicant member state. The Commission also examines the country’s capacity to incorporate the acquis communautaire and identifies the factors that could pose problems during negotiations (acquis screening). Finally, the Commission issues its recommendation on whether or not to open negotiations.

On 6 October, the European Commission published progress reports on the applicant countries Turkey, Romania and Bulgaria, as well as a pre-accession strategy for Croatia.

The negotiations with Romania and Bulgaria were completed in 2004 and the European Council of 17 December 2004 concluded that the Accession Treaty could be finalised so that it could be signed during the GAERC meeting of April 2005 on condition of agreement by the European Parliament.
On 17 March 2005 negotiations on accession will begin with Croatia on condition that the country fully collaborates with the International Criminal Tribunal for the Former Yugoslavia (ICTY). Negotiations with Turkey will start on 3 October providing that the country signs the Protocol regarding the enlargement of the customs union with the new Member States.
Furthermore the European Council also determined which elements would be taken into account for the framework in which future accession negotiations, including those with Croatia and Turkey, would take place.

The recent enlargement has political, economic and cultural benefits:
- The extension of the zone of peace, stability and prosperity in Europe will enhance the security of all its citizens;
The EU is now the world’s biggest economy, with more than 450 million people and a collective gross national product of more than €8.5 trillion. The enlargement will provide a major stimulus to economic growth in both old and new member states.
- The quality of life for citizens throughout Europe will improve as the new members adopt EU policies on environmental protection and the fight against crime, drugs and illegal immigration;
- The advent of new members will enrich the EU through increased cultural diversity, interchange of ideas and better understanding of other peoples;
- Finally, enlargement will strengthen the Union’s role in world affairs - in foreign and security policy, and trade policy.