This timely book presents conclusive new evidence on paradiplomacy in contemporary Europe, challenging mainstream understanding of how substate actors engage with the European Union (EU). Drawing on global scholarly expertise, and in a constructive dialogue with International Relations, Federal and EU Integration Studies, and Comparative politics, it reveals how political and policy engagement with the EU operate through multiple layers, influencing both substate and state behavior in today's complex international order. The volume establishes a fresh framework for understanding paradiplomacy, enriched by examining the domestic impact of independent substate activism on European policy. Through detailed analysis, it demonstrates how substate actors substantively influence the EU agenda, consequently reframing state activism and leadership within formal decision-making spaces. Breaking new ground, the collection examines not only powerful European substate actors like Catalonia, Bavaria, and Scotland, but also incorporates perspectives from non-European entities such as California and Illinois. Accessible, meticulously researched, and expertly presented, this volume is essential reading for anyone seeking to understand the dynamics of real-world paradiplomacy in our time. The chapters in this book were originally published as a special issue of Territory, Politics, Governance.