This book examines security and defence cooperation between the EU and third countries, in particular the United Kingdom and Norway. Brexit has placed the question of third-party engagement firmly back in the spotlight, especially given the UK significance as a security and defence actor, and the failure of both sides to agree terms for cooperation in this area. The book explains why the seemingly obvious need for cooperation and shared strategic interests alone does not lead to frictionless cooperation or integration between the EU and like-minded third countries. Adding a theoretical and conceptual depth to what is still largely an empirical topic, it draws important conclusions about the possibilities and limits of European security and defence cooperation during challenging times. It also raises key questions about the nature and suitability of the pre-existing security and defence architecture in Europe, and the place of non-EU members within it. The book will appeal to academics and students interested in European politics, EU security, and security and defence studies.