Book excerpt: Highlighting the challenges and prospects of European security cooperation, this volume examines the impact of Brexit on strategic aspects of security, peace, defence and foreign policy for both the European Union and the UK. It applies theoretical and methodological approaches from international relations and security studies to analyse the causal mechanisms of security cooperation, and covers topics including innovative security technologies, defence procurement, EU-NATO relations, new capabilities frameworks (such as PESCO, EDF and EII), the role of French-German military cooperation, and the implications of Brexit for European deterrence or the Northern Ireland peace process. The findings contribute to a better understanding and management of anticipated challenges and sources of instability in post-Brexit Europe.