In order to join the EU the would-be member state must submit an application for membership to the Council of the European Union, which will then ask the European Commission to assess the applicant's ability to meet the conditions of membership.
Once the Commission delivers a positive opinion and the Council unanimously agrees a negotiating mandate, negotiations are formally opened between the candidate and all the member states.
The basis of the accession negotiations is the adoption and implementation of the Acquis, which is the entire body of EU supranational law and treaties to date. This principle is now included in the Lisbon Treaty. The different areas for which reforms are needed in order to meet the accession conditions are called "chapters of the Acquis".
The candidate countries are required to adapt their administrative and institutional infrastructures and to bring their national legislation in line with all EU legislation from the foundation of the EC-EU up to the date of accession in the areas of the different chapters. These are reviewed during the screening of the Acquis and are evaluated regularly up until the time each chapter is regarded as closed.
If successful, the negotiations lead to precise terms and conditions which are then enshrined in an Accession Treaty and an Act of Accession that the would-be member states sign with the current member states.
10 of the countries which started negotiations in Athens on 16 April 2003 became EU member states on 1 May 2004. Romania and Bulgaria became EU members on 1 January 2007, Croatia on 1 July 2013.
The accession treaties state that new member states have the same rights and obligations as the original member states. Some obligations, laid down in special protocols, only enter into force at intervals after accession. So the accession negotiatons are essentially about the timing of these. The accession treaties and acts of accession have to be adopted unanimously by the existing member states and by each new member state.
There were referenda in 2004 in each of the ten new member states, except for Cyprus, in order to allow national ratification of their accession treaties. No referendum was held in Romania and Bulgaria. Croatia also had a referendum.
If a country fails to ratify, the number of votes and seats and the figures for qualified majority voting are changed proportionally or technically without new negotiations being required among all member states.
A legal base for such changes is inserted in the accession acts.
The Lisbon Treaty includes a new Article 8 TEU concerning agreements with neighbouring countries, opening the possibility of a special partnership agreement with Turkey snd possibly y others in the future.
When an accession treaty is adopted, it is ratified and sent to the Italian Republic, which collects and keeps all the instruments of ratification from the original Rome Treaty onward. The accession criteria, or Copenhagen criteria, are the essential conditions which all candidate countries must satisfy to become an EU member state .
European Commission: Directorate General for Enlargment
Summaries of legislation: Enlargement