The process of monetary integration in general is followed by a set of rules. The well known Optimum Currency Area (OCA) criteria are basically structural while official, so called Maastricht criteria for joining Economic and Monetary Union (EMU) are nominal. Many economists agree that nominal convergence itself is not sufficient for monetary union to function successfully and that also a certain level of structural convergence between the member states has to be achieved. In addition, in time of approaching eurozone enlargement the adequacy of Maastricht criteria is questionable. Whereas their modification seems to be politically unfeasible their flexible interpretation taking into account individual needs of the new member states would be desirable.