The European Union (EU) is often seen as a guarantor of peace and stability in Europe, but in the light of ‘Brexit’, shifts in transatlantic relations, the migration crisis and, growing political tensions between Member States, new questions need to be asked about what these crises and challenges entail for the Union. This chapter discusses these questions on the basis of research on security communities in international relations. The chapter describes the historical evolution of the security community concept and summarizes the main theoretical insights gained from studies conducted over several decades. The concept is used to analyse the development of the EU as a security-community-building institution, with an emphasis on military and civilian crisis management. The EU’s response to the refugee situation in the Mediterranean region is also analysed. The chapter concludes with a reflection on the extent to which the EU can act as a security-building institution beyond its borders, and this provides the basis for a set of policy recommendations highlighting that the EU should seek to strengthen practical cooperation with non-members in the field of international crisis management.