This thesis examines the explicit and implicit limits under international law on the use of private military and security companies (PMSCs) in armed conflict and peace operations. Having concluded that most PMSC contractors are civilians under international humanitarian law, and therefore should not directly participate in hostilities, it explores the relationship between that concept and the use of force in self-defence - including in peace operations - to assess what PMSCs may be contracted to do. In addition to exploring other limits on state use of PMSCs, the study analyses the principles of peacekeeping and IHL to identify limits on the roles for which PMSCs may be used, especially as a component of the peacekeeping force or as security guards. Finally, this study provides a critical analysis of the potential responsibility of states and international organisations for wrongful acts of PMSCs.