What does ASEANs twenty-year cooperation on human trafficking say about regional integration? This paper assesses ASEANs capacity to solve the regions long-lasting human trafficking problem. ASEAN is generally treated as a weak institution in one of the
most state-centric regions, and human trafficking is a difficult problem even for strong institutions due to its transnational nature and push-and-pull incentives. The approach in this paper is to find the causal mechanisms between international organizations and effective international cooperation in this least-likely case. By compiling ASEANs policy documents and examining its institutional arrangements and national implementation from 1997 to 2015, this study has two major findings. First, ASEANs human trafficking cooperation emerged as an initiative to rebuild its organizational integrity during the 1997 Asian Financial Crisis, but it has been incrementally developed and maintained through its initially non-binding agreements. Second, ASEANs cooperation on human trafficking was not merely on paper, but was implemented at the domestic level in the form of legal harmonization, which is a sign of ASEANs progress towards regional integration. By highlighting voluntary and slowly formalizing cooperation this study presents a more hopeful picture of ASEAN cooperation, and suggests a new approach to studying regional integration.