Book excerpt: This book explores the viability of future UK-EU internal security arrangements in light of Brexit, including their impact on the UK’s and the EU’s security and international standings. The authors discuss on-going negotiations and address the main political and legal concerns of possible future arrangements. As the UK prepares to leave the EU, the country is faced with having to develop new cooperation models with its neighbours to fight growing transnational security threats, as well as new strategies to maintain its leading role as an international security actor. In exploring these issues, the book aims to contribute to the general knowledge on the risks and opportunities associated with the disentanglement of the UK from European internal security cooperation; to shed more light on the debates surrounding the negotiations; and to inform the policy discussions that form the basis of proposed cooperation models and that are likely to significantly shape the future UK-EU security relationship.