Examining the economic costs and benefits of currency union has value in itself and, insofar as policymakers take non-economic costs into account, doing so helps identify costs they are prepared to accept in order to achieve political objectives. Questions of policy competence have become more complex, however, since Mundell's seminal article of 1961. Krugman provides a simple framework for relating trade integration to currency area membership. Collectively, articles in this issue of the Journal of Public Policy support the conclusion that can also be drawn from reviewing the economic literature on currency union: the costs and benefits of currency union are contingent on many circumstances with the likelihood of contextual and temporal effects.