The Law of the European Union is the unique legal system which operates alongside the laws of Member States of the European Union (EU). EU law has direct effect within the legal systems of its Member States, and overrides national law in many areas, especially in terms of economic and social policy. The EU is not a federal government, nor is it an intergovernmental organization. It constitutes a new legal order in international law for the mutual social and economic benefit of the Member States. It is sometimes classified as supranational law.

European Union law has evolved gradually over the last 56 years. When the Treaty of Paris was signed in 1951, it established the European Coal and Steel Community, and comprised just six Member States. Five years later the European Economic Community was founded by the same six Member States. As of 2007, there are around 500 million EU citizens in 27 Member States subject to EU law, making it one of the most encompassing modern legal systems in the world.

EU law has what is known as a three pillar structure. The first, oldest and most important 'pillar' deals with law concerning economic and social rights and how European institutions are set up. This is found in the Treaty of the European Communities, signed in Rome 1957 and subsequently amended by other Treaties concluded between the Member States. The second and third pillars were established under the Treaty of the European Union, signed in Maastricht 1992. The second pillar concerns the European Union Common Foreign and Security Policy (CFSP). The third pillar concerns Police and Judicial Co-operation in Criminal Matters (formerly 'Justice and Home Affairs'). Technically speaking, "EC law" denotes anything to do with the first pillar and "EU law" denotes the law regarding all three pillars.