The landscape of UK security and defence faces possibly one of the greatest changes since the end of World War Two. The UK’s exit from the EU has removed it from under the umbrella of both the EU’s Common Foreign and Security Policy (CFSP) and, in turn, the Common Security and Defence Policy (CSDP), in which it has played a role in since 1992. While at times the security and defence relationship between the UK and the EU has been strained, it has been mutually beneficial in a number of very important ways. Historically, although the UK has not always been the most active member in EU security and defence, its role as one of the two biggest military powers in the bloc has made it one of the biggest contributors of resources to CSDP missions, both militarily and diplomatically. This has greatly bolstered the EU’s military and diplomatic power at home and abroad. Its involvement in EU security and defence has also provided the UK with many benefits, The UK’s membership of the EU has enabled it to take advantage of the EU’s position as one of the biggest economic powers in the world. In turn, the UK has been able to utilise the EU’s ‘soft power’ tools—economic sanctions, humanitarian and peacekeeping capabilities—to help promote its own foreign, security and defence interests beyond the borders of the EU. Whilst the EU’s hard power capabilities will undoubtedly take a major blow because of Brexit, so too will the UK’s soft power capabilities. This chapter aims to assess, historically, what has been the most difficult and beneficial aspects of the security and defence relationship between the UK and the EU for both parties; to analyse the current position of the UK and the EU post-Brexit; to assess the main difficulties and opportunities that now present themselves for the UK and the EU post-Brexit; and to explore what a future security and defence relationship might look like between the UK and the EU.