The European Union has experienced several phases of enlargement throughout its history. The founding countries, which established the European Economic Community in 1958, were Belgium, France, Germany, Italy, Luxembourg, and the Netherlands. The first significant enlargement took place in 1973, with the accession of Denmark and Ireland. Greece joined in 1981, followed by Portugal and Spain in 1986.

A major phase of expansion occurred in 1995, when Austria, Finland, and Sweden became members of the Union. On 1 May 2004, one of the largest enlargements in European history took place, with the accession of ten countries: Cyprus, the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Malta, Poland, Slovakia, and Slovenia.

Subsequently, in 2007, the EU welcomed Bulgaria and Romania, while the most recent country to join was Croatia, in 2013. These phases of enlargement reflect the gradual political, economic, and cultural consolidation of the European Union over the decades.

Today, the enlargement process continues. The main candidate countries are Albania, North Macedonia, Montenegro, Serbia, Bosnia and Herzegovina, Kosovo, Moldova, Ukraine, and Georgia. Accession does not follow fixed deadlines, but depends on compliance with European criteria regarding democracy, the rule of law, and economic standards. Countries that are more advanced in negotiations, such as Montenegro and Albania, could join during the second half of the 2020s, following a process that includes reforms, negotiations, and final ratification by all Member States.