This paper gives a sketch of some theoretical foundations, showing the development of the Optimum Currency Areas theory. It presents OCA properties, the fulfillment of which is a precondition to consider a chosen geographic domain as an optimum currency area. It provides a background on some issues of cost and benefit analysis derived from participation in a currency area and from relinquishing monetary policy autonomy. The paper also shows the influence of theoretical and economic phenomena on reconsidering some of the OCA theory's elements. (original abstract)