The book examines the relationships between multilateral institutions that play a significant role in contemporary efforts to manage international security. The topic of "inter-organisational relations in international security" emerges from the growing trend for multilateral institutions to emphasise their ties to other such institutions, often using such links as evidence of their legitimacy, relevance and influence. While others, conversely, enter into disputes with one another over their policies, agendas, authority and control over resources. This trend has been accompanied by a growth in commentary about the politics of multilateral governance. Against this background, it is surprising that the topic of "inter-organizational relations" has been largely neglected by the academic literature on International Relations (IR). While there are abundant works on the nature, composition, institutional design, conceptions and practices of various global and regional multilateral institutions, there are few focused on the relationships between them. This book seeks to contribute to filling this gap. Drawing on the few existent IR works on the topic that note the importance of "environmental embeddedness", "inter-organizational networking" and "organisational interplay", the book adopts a relational perspective that foregrounds and emphasises the stakes, processes and outcomes of the relationships between multilateral institutions. To this end, this book adopts the established conceptual toolkit for assessing inter-organizational relations within the Organizational Studies literature. It starts from the assumption that inter-organizational relationships are politically both significant and contested (either more cooperatively or more antagonistically). Therefore, by critically investigating bilateral, trilateral and wider networks of inter-organizational relations, we can better understand the dynamics underlying the contemporary governance of international security. This book will be of much interest to students of international security, international organisations, global governance and area studies, more generally.