This work contributes to a better understanding of the growing subnational involvement in foreign affairs. It offers a general view of the most prominent aspects in the development of subnational foreign action around the world, dealing with topics such as the repercussions upon subnational autonomy of the progressive constitution of diverse international regimes like the European Union, NAFTA, and APEC, or the complex relation between the growing subnational foreign action and the contemporary conditions for the formulation and implementation of foreign policy in federal and quasifederal states.