Economists have long feared that economic convergence towards monetary union might be superficial or short-lived; more the product of political will than of economic advantage or market incentives. The worry is the convergence criteria were defined in nominal terms, and for one particular moment of time. This says nothing about convergence in structures and responses, or about the ability to remain converged over a period of time. If significant differences remain in structures, or in the national responses to policy changes, shocks, or other events, then it is inevitable that common policies — and a common monetary policy in particular — will have different impacts in different places. That could delay convergence, if not start to drive the union’s economies apart.