The euro is the monetary unit and currency of the European Union (EU). Introduced as a noncash monetary unit in 1999, euro currency notes and coins appeared in participating countries on January 1, 2002. After February 28, 2002, the euro became the sole currency of 12 EU member states, and their national currencies ceased to be legal tender.
The euro’s origins are in the Maastricht Treaty (1991), which called for a common unit of exchange and set strict criteria for conversion to the euro and participation in the Economic and Monetary Union (EMU). The 20 participating EU countries are known as the euro area, euroland, or the euro zone. The euro is also the national currency in several countries outside the EU, including Andorra and San Marino, and it is the de facto currency in Montenegro, Kosovo, and other places.