This book investigates the modern privatisation of war. It specifically focuses on the legal regime regulating private military and security company (PMSC) personnel in armed conflicts. The law regulating PMSC personnel is analysed from two perspectives. Firstly, can one of the three following legal statuses established by international humanitarian law – “mercenary”, “combatant” or “civilian” – be applied to PMSC personnel? Secondly, the book employs a context-dependent methodology to explore the legal regime regulating PMSC personnel. It argues that the legal regime regulating PMSC personnel in armed conflicts depends on who hires them: individual states, the United Nations, non-governmental organisations, or armed groups. This approach represents a departure from previous literature, where attention has primarily been paid to the use of PMSCs by states.