This document is an excerpt from the EUR-Lex website
Accession of new countries to the European Union (EU) is governed by Article 49 of the Treaty on European Union (TEU). A country wishing to join the EU needs to satisfy 2 conditions:
Having met the accession (Copenhagen) criteria, a candidate country has to demonstrate, in the course of the negotiation period, that it is able to take on the rights and obligations of EU membership. Once negotiations are successfully completed on all the 35 chapters of the EU's 'acquis', the Council must give its unanimous approval and Parliament its consent in order for an accession treaty to be signed.
This treaty is then signed by the candidate country with the individual EU countries, which then ratify it in accordance with their individual constitutional rules.