The European Union (EU) is a political and economic union of 28 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 in those matters (only) where members have agreed to act as one. 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. For travel 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 and European citizenship were established when the Maastricht Treaty was enacted in 1993. The EU traces its origins to the European Coal and Steel Community (ECSC) and the European Economic Community (EEC), established, respectively, by the 1951 Treaty of Paris and 1957 Treaty of Rome. The original members of what came to be known as the European Communities were the Inner Six: Belgium, France, Italy, Luxembourg, the Netherlands, and West Germany. The Communities 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 latest major amendment to the constitutional basis of the EU, the Treaty of Lisbon, came into force in 2009. While no member state has left the EU or its predecessors, the United Kingdom signified an intention to leave after a membership referendum in June 2016 and is negotiating its withdrawal.