This chapter systematically organizes various theories and methodologies of IR and other disciplines to explain paradiplomacy comparatively, between and within countries. Systemic neoliberal theories are used to explain growing paradiplomacy in the current international system, and domestic theories on federalism and institutional theories on veto points and players are integrated to explain the variation between and within countries. Using a typology of the intergovernmental relations in the international policy realm, ten federal countries are classified; four are inclusive (Australia, Belgium, Canada and Germany), four are complementary (Argentina, Brazil, South Africa and the USA) and two are exclusive (India and Russia). This typology, which is part of the central-local division of power, inserted into the institutional and partisan veto points and players model, is used as the structure to analyze paradiplomacy comparatively between countries.