This book provides practice-oriented insights into the agency of two previously underestimated actors in Southeast Asian regionalism: the ASEAN Secretariat and ASEAN's dialog partners. In doing so, it offers an inside view of the policymaking processes in the ASEAN Political-Security and the ASEAN Economic Community, analyzing the interplay and agency by both actors in agenda setting, formulation, decision-making, implementation, and monitoring. Drawing on a trove of novel data, including never-before analyzed sources and numerous interviews with ASEAN insiders, the book showcases a number of concrete cases of policymaking, including competition and counterterrorism policies. The chapters focusing on the ASEAN Secretariat address aspects related to institutional autonomy, capacity, and reforms within the bureaucracy. In the chapters on ASEAN's dialog partners, the book provides insights into the bilateral management of institutional support programs, as well as the impacts of support on ASEAN's policymaking processes.