Extensive prior research on the economics of European monetary union highlighted some potential risks (the known unknowns) but overlooked others (the unknown unknowns). Asymmetries among participating countries, the potentially destabilising character of a one-size-fits all monetary policy, the weakness of adjustment mechanisms, the lack of incentives for fiscal discipline, the possibility of sovereign solvency crises and their adverse consequences were all known and understood. But policymakers often relied on a complacent reading of the evidence. The potential for financial disruption was vastly underestimated. Economists generally did not consider, or underestimated, the possibility of balance of payment crises such as those experienced by southern European countries, or the risk of a feedback loop between banks and sovereigns. Remedying EMU's systemic deficiencies is on the policy agenda. Banking union would go a long way towards addressing the fault lines. The urgent question for economists is if it is going to be enough and, if not, what else should complement the 'bare-bones' EMU of Maastricht.