To be able to join the euro area the EU member states are required to fulfil the so-called 'convergence criteria'. These are economic and legal conditions agreed in the Maastricht Treaty in 1992 and are also known as 'Maastricht criteria'.
The final decision on a EU country's joining the euro area is taken by the Council of the EU, based on a proposal by the Commission and after the European Parliament has been consulted.
All EU member states have committed to introducing the euro, except for Denmark and the United Kingdom which have the so-called 'opt-out clauses' in the EU treaties, exempting them from adopting the euro. These 2 countries may nevertheless apply for membership of the euro area if they so decide.
Currently the euro area comprises 19 member states: Austria, Belgium, Cyprus, Estonia, Finland, France, Germany, Greece, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Portugal, Slovakia, Slovenia and Spain.