The arguments developed in this article are based on an economic cost-benefit analysis, or exactly the challenges and opportunities for adopting a common currency. The concepts imply that a monetary unification in Europe better suits the economic interests of the different individual countries if it can proceeds at different speeds, or if some countries that today feel it is not in their national interest to do so, have the option to wait before joining the union. Nevertheless, even the countries that are net gainers from a monetary union take a risk by joining the union. The risk is that when large shocks occur, they will find it more difficult to adjust, having relinquished their national cur- rency.