This House of Lords Library briefing has been prepared ahead of the debate on the impact of the withdrawal from the European Union on the United Kingdom’s armed forces and diplomatic service, to be held in the House of Lords on 8 December. The briefing examines current defence capabilities and cooperation between the UK and the EU. It then explores the possible future prospects for UK defence, particularly the role of the armed forces. The latter part of this briefing examines the UK’s diplomatic service, highlighting some of the issues that have arisen about the UK’s diplomatic relationship with the EU and its global role following the UK’s withdrawal. For reasons of brevity, this briefing does not examine the implications of the UK’s withdrawal on wider external relations, such as the UK’s future trade relationships with other countries in and outside the EU, or the impact on overseas development policy. Further reading on these subjects is identified in section 3.