The Treaty of Accession 2003 was the agreement between the member states of the European Union and ten countries (Czech Republic, Estonia, Cyprus, Latvia, Lithuania, Hungary, Malta, Poland, Slovenia, Slovakia), concerning these countries' accession into the EU (see 2004 enlargement of the European Union).

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.

The European Union comprises a large number of overlapping legal structures which is a result of it being defined by successive international treaties.

As from 1 November 2004, whenever the Council vote to adopt a decision by Qualified Majority, this need support both from a majority of member states which at the same time represents minimum 232 out of 321 weighted votes (72,3%), while also as a third requirement those states must represent at least 62% of the total population of the Union.