This article provides an overview of recent research into the macroeconomic costs and benefits of monetary unification. We are primarily interested in Europe's monetary union. Given that unification entails the loss of a policy instrument, its potential benefits have to be found elsewhere. Unification may serve as a vehicle for beneficial institutional changes. In particular, it may be a route toward an independent monetary policy, which alleviates the scope for political pressure to relax monetary policy. Unification also eliminates harmful monetary policy spillovers and competitive devaluations. We explore how disagreement between the monetary and fiscal authorities about their policy objectives can lead to extreme macroeconomic outcomes....