The European Union (EU) consists of 27 member states. Each member state is party to the founding treaties of the union and thereby shares in the privileges and obligations of membership. Unlike members of other international organisations, the member states of the EU have agreed by treaty to shared sovereignty through the institutions of the European Union in some (but by no means all) aspects of government.
Member states must agree unanimously for the EU to adopt some policies; for others, collective decision making is by qualified majority voting. Subsidiarity, meaning that decisions are taken collectively if and only if they can not realistically be taken individually, is a founding principle of the EU. In the 1950s, six core states founded the EU’s predecessor European Communities (Belgium, France, Italy, Luxembourg, the Netherlands, and West Germany).
“Map of the EU” remaining states have acceded in subsequent enlargements. To accede, a state must fulfill the political and economic requirements known as the Copenhagen criteria, which require a candidate to have a democratic, free-market government together with the corresponding freedoms and institutions, and respect for the rule of law. Enlargement of the Union is also contingent upon the consent of all existing members and the candidate’s adoption of the existing body of EU law, known as the acquis communautaire.
Until 2020, no member state had ever withdrawn or been suspended from the EU, though some semi-autonomous areas or dependent territories had previously left. The UK government invoked Article 50 of the Treaty on European Union on 29 March 2017 to formally initiate the Brexit process. Completion occurred on 31 January 2020 (at 23:00 London time),  with all other arrangements remained in place during a transition period, which ended at 23:00 GMT on the 31 December 2020, while future trading arrangements were discussed.