The founding countries of the EU were Belgium, Germany, France, Italy, Luxembourg and the Netherlands.
The UK, Denmark, Ireland joined in 1973, followed by Greece, Spain, Portugal, Austria, Sweden and Finland.
In 2004, the Czech Republic, Estonia, Cyprus, Latvia, Lithuania, Hungary, Malta, Poland, Slovakia and Slovenia joined.
Bulgaria and Romania became EU members in 2007 and Croatia was the last country to join in 2013.
What is the EU membership criteria?
Any European country can apply for membership as long as it respects EU’s democratic values and human rights.
It must have an economy that can cope with the EU’s market forces and be willing to accept established EU law and practice.
Support for European Union
Support for the E.U. coincides with renewed economic confidence across the continent. The euro zone economy grew significantly in 2016, a trend that's continued. Unemployment has dropped to the single digits, and job creation accelerated to a nine-year record at the beginning of this year.
Turkey and the Balkan states of Macedonia, Montenegro, Serbia and Albania are now waiting in line to join the EU.