If the United Kingdom chooses to cooperate with the EU and its member states in matters of foreign, security, and defence policy after Brexit, it will not be the only non-EU country to do so. The European Union has myriads of different types of collaborations – with countries as different as Norway and Turkey. It has also close inter-institutional links with other security organizations such as NATO or various UN institutions. This section brings together the broad literature that analyses in greater depth the different aspects of EU foreign, security, and defence policy that are relevant for the in-depth understanding of the pontential roles of the UK as non-EU member. Given that the EU is a highly institutionalized polity, a lot of emphasis is put on formal and informal dimension of EU policy-making in international affairs. Finally, it also includes a special section on what is arguably the most widely debated inter-institutional relationship of the EU, i.e. EU-NATO cooperation. Since the UK will remain a member of NATO after Brexit, this cooperation is likely to become even more important in the future.