Optimum currency area theory brings a set of criteria that can help us in answering the question which countries should share the same currency and does it pay off to fix exchange rates permanently. Over the time, the theory has proven to be misnomer, as it does not provide yes or no answer to the central question mentioned above, it only provides criteria that make currency area acceptable. Economic and monetary union is the result of progressive economic integration in the EU. It is an expansion of the EU single market, with common market regulations and free movement of goods, capital, labour and services. Some of the benefits of the euro: more choices and stable prices for consumers and citizens, greater security and more opportunities for businesses and markets, improved economic stability and growth, more integrated financial markets and a stronger EU presence in the global economy. The costs of the euro include: the loss of an independent monetary and exchange rate policy, the implementation of a unique monetary policy will not satisfy the needs of all members, the conversion of the national currency into euros can stimulate price growth, one-time conversion costs and participation in common costs. Slovenia became a member of the EU in 2004, and in 2007 it introduced the euro as a single currency. Such a rapid adoption of the euro was influenced by the adoption of the Program in 2003, the rapid entry into ERM II and the positive opinion of the EU. Today, thirteen years after the introduction of the euro, the effects on the entire Slovenian economy are visible.