|Private military and security companies do not operate in a complete legal vacuum. The inherently transnational nature of the so called market for force, however, makes traditional single state regulation insufficient. Ensuring control over the private military industry is thus a complex endeavour involving a broader network of actors alongside states, such as international and nongovernmental organizations, private customers and the industry itself. In order to unravel the challenges produced by the emergence of a PMSI and show the need for a multilayered and multifaceted approach to regulation, this paper will focus on three basic questions. First, it will analyze what to regulate, exploring the nature and the activities of PMSCs. In addition, it will explicitly focus on why the market should be regulated by drawing on the literature on the control over military force. Finally, it will concentrate on how to regulate, approaching the issue from two different standpoints. On the one hand, it will explore the main regulatory tools available to public actors, analyzing the potential of a combined approach based on both legal and informal regulation. On the other, it will look at the challenges and the potential of regulation at different levels. After analyzing the role of home, contracting and territorial states, the final sections will focus on different avenues for international regulation and on the need for EU action, whose potential in regulating the market appears huge and, to date, insufficiently exploited.