The European Union is the most developed integration among its counterparts. Focusing on the customs union at first, the European Union developed and applied common policies within time. After the expiry of the Bretton Woods System, the economic and monetary union among the European countries became inevitable. It is a great evolution for the international money system and the other countries that Euro entered into the circulation on January 1, 2002 after a long process. The international currencies of the member nations disappeared and Euro took their place as the common currency. In this study, the compatibility of the economic and monetary union to the Theory of Optimum Currency Areas is analyzed.