The basic insight from the theory of optimum currency areas (OCA) is that countries participating in regional monetary integration face a trade-off between the shared benefits of adopting a common currency with a single monetary policy and the individual costs of giving up the exchange rate as a national instrument supporting macroeconomic adjustment. The member states should ideally form a closely integrated economic region with a similar business cycle, a diversified industrial structure, and a central budget to assist economies that move out of sync. A high degree of symmetry in economic shocks, adequate flexibility of production factors and countercyclical budgetary transfers from the centre increase the likelihood that all member states will be able to reap the fruits of being part of a monetary union. Although the OCA theory does not provide a coherent identification of the economic and political criteria that candidate countries should meet, it nevertheless gained the status of mainstream theory. Over time, two challenges undermined its role as a universal compass for regional monetary integration: the crisis in the eurozone and the changing global order.