The '''euro''' is the common [[currency]] in the Eurozone (a group of 15 [[European Union]] member states with a population of 320 million), five other states with formal agreements to use it, and four others without formal agreement. Use of the euro began in January 2002, and 12 nations switched their currency immediately to the euro. There was more than €610 billion in circulation in December 2006 (worth US$802 billion at the time), hence the euro has the highest value of cash in circulation in the world, having surpassed the U.S. dollar.<ref>{{cite web| last = Atkins| first = Ralph| title = Euro notes cash in to overtake dollar| publisher = Financial Times|date=2006-12-27| url = http://www.ft.com/cms/s/18338034-95ec-11db-9976-0000779e2340.html| accessdate = 2007-05-04}}</ref> | The '''euro''' is the common [[currency]] in the Eurozone (a group of 15 [[European Union]] member states with a population of 320 million), five other states with formal agreements to use it, and four others without formal agreement. Use of the euro began in January 2002, and 12 nations switched their currency immediately to the euro. There was more than €610 billion in circulation in December 2006 (worth US$802 billion at the time), hence the euro has the highest value of cash in circulation in the world, having surpassed the U.S. dollar.<ref>{{cite web| last = Atkins| first = Ralph| title = Euro notes cash in to overtake dollar| publisher = Financial Times|date=2006-12-27| url = http://www.ft.com/cms/s/18338034-95ec-11db-9976-0000779e2340.html| accessdate = 2007-05-04}}</ref> |