Withdrawal from the European Union is a right of European Union (EU) member states under the Treaty on European Union (Article 50): "Any Member State may decide to withdraw from the Union in accordance with its own constitutional requirements."
Greenland, part of the Danish Realm, voted to leave the EU's predecessor, the European Economic Community (EEC), in 1985. Algeria left upon independence in 1962, having been a part of France until then.
The United Kingdom held a referendum on 23 June 2016 to decide whether it should remain a member of the European Union or leave it, which was won by the "Leave" vote. The voting results showed 51,9% voted to leave (a margin of 3,8% above those voting to remain) and a voting turnout of 72,2%.
Prior to this referendum, the United Kingdom had held a referendum in 1975 on withdrawal from its predecessor, the EEC; 67,2% of voters chose to remain in the Community with a turnout of 64,62%.
The Treaty of Lisbon introduced an exit clause for members who wish to withdraw from the Union. Under TEU Article 50, a Member State would notify the European Council of its intention to exit the Union and a withdrawal agreement would be negotiated between the Union and that State.
The treaties of the European Union would cease to be applicable to that State from the date of the agreement or, failing that, within two years of the notification unless the Council, in agreement with the State, unanimously decides to extend this period. The two year period of time in which the terms of the withdrawal agreement are negotiated is known as the sunset period.
The agreement is concluded on behalf of the Union by the Council and must set out the arrangements for withdrawal, including a framework for the State's future relationship with the Union.
