The European Union is a political and economic union of 27 member states that are located primarily in Europe. It has an area of 4,475,757 km2 (1,728,099 sq mi), and an estimated population of over 510 million. The EU has developed an internal single market through a standardised system of laws that apply in all member states. EU policies aim to ensure the free movement of people, goods, services, and capital within the internal market, enact legislation in justice and home affairs, and maintain common policies on trade, agriculture, fisheries, and regional development. Within the Schengen Area, passport controls have been abolished. A monetary union was established in 1999 and came into full force in 2002, and is composed of 19 EU member states which use the euro currency.

The EU operates through a hybrid system of supranational and intergovernmental decision-making. The seven principal decision-making bodies—known as the institutions of the European Union—are the European Council, the Council of the European Union, the European Parliament, the European Commission, the Court of Justice of the European Union, the European Central Bank, and the European Court of Auditors.

The EU traces its origins from the European Coal and Steel Community (ECSC) and the European Economic Community (EEC), formed by the Inner Six countries in 1951 and 1958, respectively. The community and its successors have grown in size by the accession of new member states and in power by the addition of policy areas to its remit. The Maastricht Treaty established the European Union in 1993 and introduced European citizenship. The latest major amendment to the constitutional basis of the EU, the Treaty of Lisbon, came into force in 2009.

The EU as a whole is the largest economy in the world. Covering 7.3% of the world population, the EU in 2016 generated a nominal gross domestic product (GDP) of 16.477 trillion US dollars, constituting approximately 22.2% of global nominal GDP and 16.9% when measured in terms of purchasing power parity. Additionally, 27 out of 28 EU countries have a very high Human Development Index, according to the United Nations Development Programme. In 2012, the EU was awarded the Nobel Peace Prize. Through the Common Foreign and Security Policy, the EU has developed a role in external relations and defence. The union maintains permanent diplomatic missions throughout the world and represents itself at the United Nations, the World Trade Organization, the G7, and the G20. Because of its global influence, the European Union has been described as an emerging superpower.

This Research Guide is intended as a starting point for research on the European Union. It provides the basic materials available in the Peace Palace Library, both in print and electronic format. Handbooks, leading articles, bibliographies, periodicals, serial publications and documents of interest are presented in the Selective Bibliography section. Links to the PPL Catalogue are inserted. The Library's subject heading (keyword) European Union is instrumental for searching through the Catalogue. Special attention is given to our subscriptions on databases, e-journals, e-books and other electronic resources. Finally, this Research Guide features links to relevant websites and other online resources of particular interest.