This book looks at the past, present and possible future relationship between the EU and the UK in the fields of law enforcement and judicial cooperation in criminal matters. It examines successively the EU-UK relationship prior to 1 February 2020; the relationship during the transition period; the relationship after the end of the transition period. The book analyses the relevant provisions of the Withdrawal Agreement, the Political Declaration, of the EU and UK negotiating mandates and draft legal texts, and the state of play of the negotiations. It looks at the possible forms that the future cooperation can take and the likely areas, which might be covered, such as cooperation with Europol and Eurojust; criminal records; DNA, dactyloscopic and vehicle registration data; passenger name records; surrender procedures, and mutual legal assistance. It also analyses the overarching issues of protection of personal data and the future role of the Court of Justice of the EU. Finally, this book puts forward some ideas on the possible impact of Brexit on security cooperation within wider Europe and on the possible emergence in future of a European Security Union within wider Europe. The volume is aimed at practitioners and academics in European Studies, International Relations, and Law.