Nothing is more important to Europe’s future as a security actor than supplying its armed forces with modern weaponry. Because individual states lack the research and development budgets and scale economies to remain autarkic, the survival of Europe’s defence-industrial base depends on international cooperation. As in other areas of international affairs, the ability of states to cooperate ‘under anarchy’ is inextricably tied to the existence of international institutions. However, the nature of arms production renders the design of institutions particularly challenging. Problems lie in both the multiplicity of potential cooperative outcomes and the variety of policy tools available. Ultimately, the choice of policies and policy tools can generate friction between the key groups of actors involved in defence-industrial policymaking. This study systematically explores how variations in the structure of international armaments institutions have shaped both the influence of different groups of actors and the nature of collaborative weapons projects. To preview my conclusions, three broad trends can be observed in the evolution of armaments institutions. These are as follows: (1) the gradual incorporation of a larger number of actors into the arms cooperation process; (2) the incremental exclusion of military professionals from armaments institutions; and (3) the growing influence of corporate actors.