The thesis titeled, "Euro - the common currency of the European Union," explores what the euro is and why it is important as a currency within the European Union (EU), as well as globaly. The euro is the official currency of 19 out of the 27 EU member states and is used in everyday business and trade throughout the eurozone. The paper explains in detail the criteria that EU members must meet to become part of the Economic and Monetary Union (EMU), which involves the use of the euro as a common currency. These criteria include price stability, sustainable public finances, exchange rate stability, and low inflation. The paper analyses the advantages and disadvantages of EU member states joining the EMU. Advantages include simplified business operations, reduced risks of currency fluctuations, and a competitive edge in the global market. On the other hand, disadvantages include the loss of monetary sovereignty and limited ability to adjust to economic shocks. Currently, 19 out of 27 EU member states are part of the EMU, while the others retain their national currencies. The prospects of EMU expansion depend on meeting economic convergence criteria and the political will of member states. Despite challenges, the euro remains a key element of European integration and stability within the EU.