The seven institutions of the European Union (EU) are seated in four different cities, which are Brussels (Belgium), Frankfurt am Main (Germany), Luxembourg (Luxembourg) and Strasbourg (France), rather than being concentrated in a single capital city. The EU agencies and other bodies are located all across the union, but usually not fixed in the treaties. The Hague is the only exception, as the fixed seat of the Agency for Law Enforcement Cooperation (Europol). Luxembourg City is the EU capital that can lay claim to having the most of the seven EU institutions based wholly or partly upon its territory, with only the European Council and European Central Bank not having a presence in the city. Over the years, Brussels has become the EU's political hub, with the College of the Commissioners — the European Commission's politically accountable executive — and the European Council both meeting at their Brussels-based headquarters, and the European Parliament and Council of the EU holding the majority of their meetings annually within the city. This has led media to describe it as the de facto "capital of the EU."

The seats have been a matter of political dispute since the states first failed to reach an agreement at the establishment of the European Coal and Steel Community in 1952. However, a final agreement between member states was reached in 1992, and later attached to the Treaty of Amsterdam.

Despite this, the seat of the European Parliament remains controversial. The work of Parliament is divided between Brussels, Luxembourg City and Strasbourg, which is seen as a problem due to the large number of MEPs, staff, and documents which need to be moved. As the locations of the major seats have been enshrined in the treaties of the European Union, Parliament has no right to decide its own seat.

Locating new bodies is also not without political disputes. The European Central Bank's (ECB) seat had to symbolise its independence from political control, and was located in a city which did not already host a national government or European institution. Some new agencies have also been based in eastern Europe since 2004 to balance their distribution across the EU.

The treaties of the European Union outline the locations of the following institutions: The European Parliament has its seat in Strasbourg, hosting twelve monthly plenary sessions (including budget session). Brussels hosts additional sessions and committees (including being used in September 2008 for normal sittings when the Strasbourg chamber was damaged). Luxembourg City hosts the Secretariat of the European Parliament. The Council of the European Union has its seat in Brussels, except during April, June, and October, when meetings are held in Luxembourg City.

The European Commission also has its seat in Brussels, although some departments and services are hosted by Luxembourg City. The Court of Justice of the European Union, (along with its component courts, the European Court of Justice and the General Court), the Court of Auditors and the European Investment Bank are based in Luxembourg City. The Economic and Social Committee and the Committee of the Regions are entirely based in Brussels while the European Central Bank is based in Frankfurt and Europol is the only agency to have its seat fixed by the treaties, in The Hague.

Separately, the Treaty of Nice establishes Brussels as the venue for all formal summits of the European Council after 2004. However some extraordinary sessions are sometimes held outside the city. The European Council's seat is however fixed by a declaration rather than a protocol, meaning it is less binding than the other seats.

The 2011 Treaty Establishing the European Stability Mechanism (ESM) states that the seat of the ESM must be in Luxembourg.