This talk discusses the implications of Brexit for the two most important security institutions in Europe, the EU’s Common Foreign and Security Policy (CFSP) and the North Atlantic Treaty Organization (NATO). How will Brexit affect the inner mechanics of the CFSP and NATO? What role will the UK play with respect to both institutions? What are the EU’s post-Brexit policy options? How will Brexit influence the broader international security environment? Using both predictive and normative arguments, the talk develops scenarios for an event that continues to be a source of much uncertainty in the global community. The talk is based on the speaker’s most recent book, The End of European Security Institutions? (Springer, 2020).