The 2009 Lisbon Treaty introduced an exit clause into the EU treaties allowing countries to leave the bloc. Under Article 50 of the Treaty on European Union, "any Member State may decide to withdraw from the Union in accordance with its own constitutional requirements."
A country would be allowed to leave the EU after notifying the other countries and negotiating an agreement on its relations with the Union. If a new agreement could not be reached the country would be deemed to have left the EU two years after giving its notice.
No member state has ever left the EU, although Greenland left the EEC in 1985 after holding a referendum.
Under the terms of the treaties, all EU countries barring the UK and Denmark, are required to join the euro. However, there is no legal mechanism for a country to withdraw from the eurozone, despite speculation during the height of the sovereign debt crisis that Greece and possibly other countries might be forced to leave the single currency. There is also no mechanism for leaving the Schengen agreement which allows for passport-free travel.
Arnie's observation
I arrived in Scotland as "a registered alien" in September 1971, two years before Great Britain joined the former "EEC".
