I discuss possible problems engendered by loss of national monetary policies, and study them from three empirical perspectives. First, are business cycles sufficiently synchronized across EMU member countries? The evidence suggests that economic activity in those countries has become increasingly correlated in the 1990s, and that policy co–ordination has played a role in generating that outcome. Second, are there asymmetries in the mechanisms through which policy affects economic activity? The paper documents that policy transmission was indeed heterogeneous in the member countries, and that structural and financial factors were sensibly related to cross–country differences in the response of output to a monetary policy shock. Third, how is policy implemented in an environment of diverse business cycle fundamentals and transmission mechanisms? Estimation of monetary policy reaction functions finds that the European Central Bank is closer to an aggregate of the central banks in Germany, France, and Italy than to the Bundesbank alone.