The United Kingdom has been one of the most powerful EU member states in terms of military capability, intelligence gathering, and diplomatic clout. But at the same time it has also been one of the most reluctant member states in terms of foreign, security, and defence cooperation in the framework of the European Union. Therefore, any assessment of the impact of Brexit in this area requires a thorough knowledge of what the existing academic literature tells us about the UK’s role in European foreign, security, and defence policies. Three concrete areas are particularly pertinent for our understanding of that role. First, the literature on British strategy and doctrine reveals the extent to which the EU may – or may not – play an important role in British security and defence thinking and planning. Likewise, this body of literature sheds light on the British influence on EU strategies. Second, the abundant literature on concrete EU policies can clarify to what extent the United Kingdom has actually contributed to the development of these policies, for instance to the transatlantic relationship. Third, the literature on 'Europeanization' reveals the extent to which the UK’s involvement in European foreign, security, and defence policies has integrated the UK with its EU partners.