At a 1998 conference, the 12 European nations agreed to adopt the euro as their single currency.
The move was part of Economic and Monetary Union, developed through the 1992 Maastricht Treaty. In May 1998, European leaders selected the countries ready to enter the euro’s third stage. Technically, 11 countries were initially approved to launch the currency in 1999; Greece joined later, bringing the number to 12 when euro notes and coins arrived in 2002.
The euro became an official currency for electronic payments and accounting on January 1, 1999. National banknotes and coins continued circulating temporarily before being replaced by euro cash three years later.
A common mix-up is confusing the euro with the ECU, the earlier European Currency Unit used as an accounting basket. The euro replaced the ECU, but it was a genuine currency rather than merely an exchange-rate reference.