This Palgrave Policy Essential considers the security and defence consequences of Brexit for the UK and the European Union. It considers the place of security and defence in the debates leading up to the Brexit referendum as well as in its follow-up. Importantly, the book also traces recent developments in the EU towards what may become a European Defence Union and, in light of this, considers how realistic the UK's aspirations for a unique relationship with the Union are. The book includes a critical analysis of the consequences of Brexit for crisis management, internal security and defence industries in the UK and EU. It concludes with an examination of the options and legal tools available to both parties as they frame their post-Brexit security and defence relations.