The past two decades have witnessed the rapid proliferation of private military and security companies (PMSCs) in armed conflicts around the world, with PMSCs participating in, for example, offensive combat, prisoner interrogation and the provision of advice and training. The extensive outsourcing of military and security activities has challenged conventional conceptions of the state as the primary holder of coercive power and raised concerns about the reduction in state control over the use of violence. Hannah Tonkin critically analyses the international obligations on three key states -...