Ten new countries joined the EU on 1 May 2004, boosting its population by almost 75 million people and fuelling a new era of prosperity.
ADVERTISEMENTThe EU put on a colourful display to celebrate Wednesday's 20th anniversary of the biggest enlargement in its history, known as the "Big Bang".
The buildings of the European Commission in Brussels, the European Parliament in Strasbourg and the EU building in Luxembourg all lit up in blue and yellow, projecting the slogan "20 Years Together".
Ten new countries joined the bloc on 1 May 2004. Almost all of them were from eastern Europe.
The integration of Cyprus, Czech Republic, Estonia, Lithuania, Latvia, Hungary, Malta, Poland, Slovakia and Slovenia brought the number of member states from 15 to 25.
The move increased the bloc's population by almost 75 million people overnight, making it one of the world's largest single markets.
Countries that joined the EU in 2004 have since enjoyed substantial economic growth.
The economies of Poland and Malta, in particular, more than doubled, while Slovakia grew by 80%, according to EU data.
Since the 2004 enlargement, around 26 million jobs have emerged in the EU. Six million of them were created in the 10 new EU countries.
The bloc's expansion also allowed an additional 2.7 million young people to take part in the Erasmus+ programme.