The euro is the official currency of the European Union (EU) and an integral part of its political and economic integration. Adopted by 19 of the EU's 27 member nations, it is the world's second most widely held and traded currency after the U.S. dollar. The euro can be divided into hundredths, or cents, and is issued in seven different denominations of euro banknotes and eight denominations of euro coins.

The European Union's introduction of the euro, which replaced many local currencies, was intended to foster closer economic and political ties among the union's member countries. In theory, the euro should also help promote a more open and efficient European market by providing businesses and consumers with a unified form of payment, reducing the cost and confusion of dealing with multiple regional currencies.

The euro has had a major economic and currency impact on the EU since its adoption. By allowing European countries to use a common currency, the euro has helped business owners, traders and consumers to conduct transactions within the Union more easily. In the end, the euro has led to more trade, increased foreign investment, and increased cross-border cooperation in several areas, from tourism and transport to university exchanges and research grants.

In spite of its success, many argue that the euro’s adoption has caused some of the EU's economic woes. Without a fiscal union to back up its monetary union, the euro has increased the risk of a currency crisis, with weaker nations being more susceptible to economic shocks. In addition, the euro has enabled irresponsible borrowing by some of its members, leading to a surge in their levels of national debt. The European sovereign debt crisis, which was triggered by the global recession of 2008, has been blamed on the euro's introduction without a fiscal union in place to manage the shared currency.

Overall, the euro has been an important part of the European Union since its introduction. While it has faced its fair share of criticism and remains vulnerable to economic shocks, the euro has helped to facilitate trade and investment between EU members and has acted as an important instrument of furthering European integration.