The tiny Baltic state of Estonia became the first former Soviet republic to adopt the euro.
The changeover was at midnight, marking the beginning of the end of the Estonian kroon.
The switch comes at a time of crisis with Europe's common currency, after two members – Greece and Ireland – required emergency bail-outs to prevent their economies from slipping into insolvency.
This is why many feel that the inclusion of Estonia, whose 12bn economy is dwarfed by the euro's total annual output of about 8 trillion, is of symbolic importance.
European Commission President Jose Manuel Barroso welcomed the country of 1.3 million to the eurozone.
"Estonia's entry means that over 330 million Europeans now carry euro notes and coins in their pockets," Mr Barroso said in Brussels. "It is a strong signal of the attraction and stability that the euro brings to member states of the European Union."
Estonia still has painful structural reforms to implement before reaching western European living standards. The country will be the 17th and poorest member of the eurozone.
Prime Minister Andrus Ansip was to launch the changeover by making one of the first bank-machine withdrawals just after midnight.