The European Union (EU) is a political and economic union of 27 member states that are signatories to the founding treaties of the union and thereby share in the privileges and obligations of membership. They have agreed by the treaties to share their own sovereignty through the institutions of the European Union in some, but not all, aspects of government. State governments must agree unanimously in the Council for the union to adopt some policies; for others, collective decisions are made by qualified majority voting. These obligations and sharing of sovereignty within the EU (sometimes referred to as supranational) make it unique among international organisations, as it has established its own legal order which by the provisions of the founding treaties is both legally binding and supreme on all the member states (after a landmark ruling of the ECJ in 1964). A founding principle of the union is the principle of subsidiarity, meaning that decisions are taken collectively if and only if they cannot realistically be taken individually.

Another notable and unique feature of membership are the commissioners of the European Commission, who are appointed by each of the governments of the member states but do not represent their member state but instead work collectively in the interests of all the member states.

In the 1950s, six core states founded the EU's predecessor European Communities (Belgium, France, Italy, Luxembourg, the Netherlands, and West Germany). The remaining states have acceded in subsequent enlargements. To accede, a state must fulfil the economic and political 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.

The United Kingdom, which had acceded in 1973, ceased to be an EU member state on 31 January 2020. No other member state has ever withdrawn from the EU and none has ever been suspended, although some dependent territories or semi-autonomous areas have left.