A common currency is an excellent monetary arrangement under some circumstances, a poor monetary arrangement under others. Whether it is good or bad depends primarily on the adjustment mechanisms that are available to absorb the economic shocks and dislocations that impinge on the various entities that are considering a common currency. Flexible exchange rates are a powerful adjustment mechanism for shocks that affect the entities differently. It is worth dispensing with this mechanism to gain the advantage of lower transaction costs and external discipline only if there are adequate alternative adjustment mechanisms.