Historically, the EU Common Security and Defence Policy (CSDP) has not been the sole framework for defence cooperation between Member States. Indeed, several ad hoc multilateral agreements coexist with measures adopted in the CSDP framework. The paper maps the relevant practice, with a focus on command and control structures, and examines the relationship between these forms of cooperation and EU law. Based on a comparison with other policy areas, the paper argues that the incorporation of EUROCORPS and other intergovernmental defence cooperation initiatives within the EU legal order would not require Treaty amendment. In this regard, it considers whether Permanent Structured Cooperation (PESCO), thanks to its modular structure, could provide an adequate frame to absorb such projects into CSDP.