The Eurozone is comprised of the countries that use the euro in place of their former national currencies. The founding members of the Eurozone are Austria, Belgium, Finland, France, Germany, Ireland, Italy, Luxembourg, the Netherlands, Portugal and Spain. Greece, Cyprus, Slovenia, Malta, Slovakia and Estonia joined later on in that order.
Denmark and the United Kingdom received opt-outs and are never required to join the Eurozone. Sweden, Lithuania, Latvia, Poland, Czech Republic, Hungary, Romania and Bulgaria are required to join in the future, though there is no deadline for their adoption of the euro.