This book studies inter-organisational relations from a new angle: power. Drawing on examples that highlight how the EU relates to NATO and to the UN, it shows how consequential inter-organisational relations are for the functioning and nature of the organisations, and how important it is to detect the forms of power exerted in these relations. Power, for international organisations, is above all about relevance. In an era when the legitimacy and role of international organisations is increasingly questioned, the organisations have a growing concern for ensuring their continued relevance. Subsequently, the management of relevance is a central part of inter-organisational relations and becomes visible in the way organisations handle questions about their tasks, hierarchies and image. Clear and accessible, the book will appeal both to the growing scholarly community working on inter-organisational relations and to a variety of audiences including practitioners and scholars outside the field of international relations.