Regime complexity, which is characterized by overlap between international organizations (IOs) concerning both policy competencies and member states, has been increasing over time. It is a defining feature of today's international system. As the regime complexity literature points out, overlaps between IOs carry potential negative effects, such as duplicated efforts or incompatible norms. This book argues that IOs can actively manage regime complexity and potentially avoid negative side effects or even create positive benefits. Yet, overlapping IOs differ in how they react. To explain under what conditions IOs disregard overlaps or manage them by resorting to confrontation or collaboration, this book addresses the following research questions: Why do organizations differ in their responses to overlaps? Why do some opt for disregard while others choose confrontation or engage in collaboration? These questions are answered by studying a subset of IOs, namely regional international organizations (RIOs), which recruit their member states on the basis of geographic criteria. It introduces a novel theoretical selection model on three junctures: saliency, ideological fit, and contextual uncertainties. This influences whether overlapping RIOs disregard one another and do not actively manage regional regime complexity (low saliency), when they choose confrontation (high saliency but low ideological fit) and when they opt for one of two ways to engage in collaboration, namely coordination (high saliency, high ideological fit, limited contextual uncertainty), or cooperation (high saliency, high ideological fit, high contextual uncertainty). The corresponding hypotheses are comprehensively analyzed in qualitative case studies from Africa, the Americas, Asia, and Europe.Transformations in Governance is a major academic book series from Oxford University Press. It is designed to accommodate the impressive growth of research in comparative politics, international relations, public policy, federalism, and environmental and urban studies concerned with the dispersion of authority from central states to supranational institutions, subnational governments, and public-private networks. It brings together work that advances our understanding of the organization, causes, and consequences of multilevel and complex governance. The series is selective, containing annually a small number of books of exceptionally high quality by leading and emerging scholars.The series is edited by Liesbet Hooghe and Gary Marks of the University of North Carolina, Chapel Hill, and Walter Mattli of the University of Oxford.