What explains why international and regional organizations in some cases choose to cooperate during peacekeeping operations, while in other cases find themselves competing for resources and control? This thesis seeks to explain variation in coordination, competition, and cooperation between international and regional organizations in the area of peacekeeping. In the post-Cold War era, a number of factors—including the proliferation of increasingly capable organizational actors, expansion of mandated tasks, and increasing complexity of conflict—have led to the development of an international peacekeeping “regime complex.” This complex is characterized by multiple international institutions that exhibit overlapping membership, are actively involved in matters of peace and security, and are connected by normative and operative interaction, both official and ad hoc. In some cases, this complex functions smoothly, while in others, it does not. By examining materialist, dependency, and identity factors at work in the peacekeeping regime complex, this thesis explores institutional interaction and the drivers of both rivalry and collaboration in the context of four cases: Democratic Republic of the Congo, Sudan, Mali, and Somalia. I hypothesize that organizations will cooperate when they hold complementary understandings of their roles within the peacekeeping regime complex, but will compete when these identities clash and overlap. Understanding these dynamics will not only lead to recommendations for more effective and efficient peacekeeping operations, but also contribute more generally to the growing theoretical field of regime complexity in international relations.