The study of European integration has in the past been plagued by the so-called sui generis problem: 'the EU is considered somehow beyond international relations, somehow a quasi-state or an inverted federation, or some other locution'. This chapter suggests one way of seeking theoretical parsimony without sacrificing the defining empirical knowledge which has been generated about Common Foreign and Security Policy (CFSP) over the years. It argues that while the sui generis nature of CFSP presents an acute problem for international relations theory, it is less pronounced with regard to traditional integration theory. The chapter concludes that traditional neo-functionalist integration theory, while in some respects problematic when applied to intergovernmental cooperation, nevertheless provides the most promising basis for further theorising about CFSP. The key features of the original European political cooperation (EPC) framework are present in the provisions of the CFSP, despite the introduction of a number of 'federal detonators'.