The Treaty of Accession is signed in Athens admitting ten new member states to the European Union.

The Treaty of Accession 2003 was the agreement between the member states of the European Union and ten countries, concerning these countries' accession into the EU. At the same time it changed a number of points which were originally laid down in the Treaty of Nice. The treaty was signed on 16 April 2003 in Athens, Greece and it entered into force on 1 May 2004, resulting in enlargement of the European Union with 10 states.