Our understanding of optimum currency area (OCA) properties has changed over time. While all properties can now be discussed in detail, the challenge of weighing and reconciling them remains. Looking ahead, we may be confronted with two distinct paradigms: specialization versus 'endogeneity of OCA' that may give rise to a paradox if taken to the extreme. The merits of OCA theory as an organizing device and a research catalyst are underlined, but its limitations are also revealed. Also noted here is the complementarity between empirical studies of OCA properties and progress toward EMU. Copyright 2005 Blackwell Publishing Ltd.