Answer

The EURO is the single currency of participating member states of the economic and monetary union as contemplated in the Treaty on European Union. It was first adopted in 1999 by 11 nations as the alternative currency in noncash transactions. In 2002, the Euro fully replaced existing currencies in 12 European Union nations, before being adopted more widely. 

Europa.eu is the official website for the European Union. 

You may want to consult the NYU Guide on European Studies.