This book discusses Brexit’s implications for the two most important security institutions in Europe, the EU’s Common Foreign and Security Policy (CFSP) and the North Atlantic Treaty Organization (NATO). While Brexit is still unfolding, this book asks what it would mean for the future embedding of the UK into CFSP and NATO, as well as how it will most likely affect the inner mechanics of the transatlantic alliance (NATO) and CFSP in particular, in the years to come. The book is divided into two parts. Part I provides a historical overview of the evolution of the relationships between the UK and NATO and the EU, respectively. Part II discusses the geopolitical contexts and potential impacts of Brexit, focusing on the contemporary security environment, as well as the options that the EU has, in the event an agreement is concluded. Using both predictive and normative arguments, this book provides likely scenarios for an event that continues to be a source of much uncertainty for the global community. Making an important contribution to one of the most important policy debates in international security affairs today, this book is of interest to students and researchers of international security affairs, European politics, and global governance as well as policymakers and practitioners working on the Brexit file.