This handbook provides an innovative, comprehensive, and authoritative examination of how subnational governments, regions, and municipalities participate in and shape the field of International Relations (IR).

Covering the conceptual, theoretical, methodological, institutional, and practical dimensions of paradiplomacy, it is divided into five parts. Part I focuses on paradiplomacy as a distinct field of research, its conceptual evolution, and its theoretical foundations through classical and alternative theories of IR. Part II presents methodological approaches, including comparative, quantitative, qualitative, and legal studies, as well as multi-level and polycentric governance analyses. Part III examines institutional aspects, discussing the roles of regions, cities, networks, cross-border cooperation, protodiplomacy, identity, and substates' involvement in international organizations. Part IV explores the intersection of paradiplomacy with public policy, covering topics such as environment, climate change, migration, urban management, culture, trade, finance, economic development, security, and decentralized international cooperation. Part V provides a detailed worldwide regional analysis, with case studies from Europe, the Americas, Asia and the Pacific, and Africa.

Offering a truly global perspective provided by experienced researchers from all over the world, this handbook will appeal to a broad audience, including scholars and students of International Relations, Urban and Regional Studies, Political Science, Public Policy, Comparative Politics, Diplomacy, Foreign Policy, Governance, and Federalism.