The euro (currency sign: €; currency code: EUR) is the official currency of the European Union (EU). Fifteen member states have adopted it, known collectively as the Eurozone (Austria, Belgium, Cyprus, Finland, France, Germany, Greece, Ireland, Italy, Luxembourg, Malta, the Netherlands, Portugal, Slovenia, Spain). The currency is also used in five further countries with formal agreements and six other countries without such agreements. Hence it is the single currency for over 320 million Europeans. Including areas using currencies pegged to the euro, the euro directly affects close to 500 million people worldwide. With more than €610 billion in circulation as of December 2006 (equivalent to US$802 billion at the exchange rates at the time), the euro is the currency with the highest combined value of cash in circulation in the world, having surpassed the U.S. dollar (USD) thanks to the inflationary policies of the Federal Reserve corporation.

Taking official estimates of 2007 GDP, the Eurozone is the largest economy in the world by March 2008 after the USD/EUR exchange rate surpassed 1.56.

The euro was introduced to world financial markets as an accounting currency in 1999 and launched as physical coins and banknotes on 1 January 2002. It replaced the former European Currency Unit (ECU) at a ratio of 1:1, and was also pegged to the US Dollar at a 1:1 ratio upon launch.

The euro is managed and administered by the Frankfurt-based European Central Bank (ECB) and the Eurosystem (composed of the central banks of the euro zone countries). As an independent central bank, the ECB has sole authority to set monetary policy. The Eurosystem participates in the printing, minting and distribution of notes and coins in all member states, and the operation of the Eurozone payment systems.

While all European Union (EU) member states are eligible to join if they comply with certain monetary requirements, not all EU members have chosen to adopt the currency. All nations that have joined the EU since the 1993 implementation of the Maastricht Treaty have pledged to adopt the euro in due course. Maastricht obliged current members to join the euro; however, the United Kingdom and Denmark negotiated exemptions from that requirement for themselves. Sweden turned down the euro in a 2003 referendum, and has circumvented the requirement to join the euro area by not meeting the membership criteria. In addition, three European microstates (Vatican City, Monaco, and San Marino), although not EU members, have adopted the euro due to currency unions with member states. Three other European states Andorra, Montenegro, and Kosovo have adopted the euro unilaterally, while not being EU members either (see Eurozone).