The expansion of European Union (EU) foreign policy cooperation since 1970 presents a number of puzzles for theorists of regional integration and International Relations. It is not directed by supranational organizations, does not involve bargaining over policy alternatives, and is not dominated by the largest EU states. Nor do the EU's common foreign policy decisions reflect 'lowest common denominator' preferences. Instead, cooperation has been achieved through decentralized institutional mechanisms, involving processes associated with both intergovernmental and social constructivist theories. This article first explains how changes in institutional context-in terms of intergovernmental, transgovernmental and supranational proceduresaffect the propensity for cooperation. It then links processes of institutionalization to an expansion of foreign policy cooperation among EU member states. Finally, it explores three policy areas (the Middle East, South Africa and nuclear non-proliferation) where EU states have adjusted their national foreign policies in line with EU foreign policy norms. KEY WORDS ♦ Common Foreign and Security Policy ♦ European Union foreign policy ♦ European Political Cooperation ♦ institutionalization ♦ international cooperation The Maastricht Treaty on European Union clearly marks a new era in the theory and practice of European integration. Much of the scholarly attention in this area focuses on the European Union's (EU) socioeconomic policy domains, primarily in terms of developing the single European market (Stone Sweet and Sandholtz, 1998). 1 This achievement, however, tends to overshadow an increasingly prominent area of European integration-the pursuit of a common foreign policy. Foreign policy cooperation has been