There are other countries outside the European Union which use the euro as well , but these are not officially in the eurozone . The eurozone can also be taken informally to include third countries that have adopted the euro , for example Montenegro ( see details on these countries below ) . Three European microstatesâ '' Monaco , San Marino and the Vatican Cityâ '' have concluded agreements with the European Union permitting them to use the euro as their official currency and mint coins , but they are neither formally part of the eurozone nor represented on the board of the European Central Bank .