What are the consequences of increasing regime complexity and institutional proliferation on global governance? Does the growing density and overlaps among institutions facilitate or hinder the ability of states to manage transnational threats through cooperation? This article argues that the impact of regime complexity on the effectiveness of cooperation depends not only on the nature of spillovers among overlapping regimes, but also on the cross-institutional strategies of states and non-state actors. I distinguish between two types of strategies through which actors can take advantage of institutional overlaps: (1) "opportunistic" or non-cooperative attempts by states to bypass legal commitments, which tend to undermine the goals of cooperation; and (2) "cooperative" strategies by intergovernmental organizations, civil society organizations, and other principled actors. These actors also engage in regime shifting, forum linking, and other cross-forum strategies to promote their institutional mandates and normative agendas. To probe the plausibility of these theoretical claims, I focus on the case of the anti-trafficking in persons regime. The overlaps between anti-trafficking laws and the migration, labor, and human rights regimes illustrate the different ways in which intergovernmental organizations and their allies in society can exploit institutional overlaps to promote greater cooperation and expand the regime complex.