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.

Athens is the capital and largest city of Greece. A significant coastal urban area in the Mediterranean, Athens is also the capital of the Attica region and is the southernmost capital on the European mainland. With its urban area's population numbering over 3.6 million, it is the eighth-largest urban area in the European Union (EU). The Municipality of Athens, which constitutes a small administrative unit of the entire urban area, had a population of 643,452 (2021) within its official limits, and a land area of 38.96 km² (15.04 sq mi).

The European Union (EU) is a supranational political and economic union of 27 member states that are located primarily in Europe. The union has a total area of 4,233,255 km² (1,634,469 sq mi) and an estimated population of over 450 million as of 2025. The EU is often described as a sui generis political entity combining characteristics of both a federation and a confederation.