There are several differences between countries which cause difficulties in forming a monetary union. The costs derive from the fact that the joining nation would not be able to change the price of its currency or to determine the quantity of national money in circulation in order to correct these differences.Theory of "Optimum Currency Areas" has concentrated on the cost side of the cost-benefit ' analysis. In practice the options for the future consists in moving towards a full monetary union. A. common currency would be required to be maneged by a common central bank.In such case nobody can suggest that, then there would be no problems. However mainly the inflationary discipline is thought to be the most important determinant in solving such problems.