This thesis analyzes whether and how international law can ensure that states relying on Private Military or Security Companies [PMSCs/contractors] to provide certain coercive services in armed conflict or occupation, cannot escape their international obligations arising out of International Humanitarian Law [IHL] and Human Rights Law [HRL]. The study focuses on the most pervasive and dangerous services, namely combat, guarding and protection, and interrogation and detention. Based on a systematic comparative analysis I identify a gap between a state's responsibility for conduct of national soldiers and equivalent conduct of contractors it hires. I argue that this responsibility gap can be bridged by applying the specific norms of attribution qua membership in the armed forces to contractors providing coercive services. Moreover, positive obligations, contained in IHL and international and regional human rights documents, contribute to ensuring state responsibility, where problems such as the extraterritorial application of HRL documents can be overcome. The thesis is divided into 8 Chapters in three Parts. In Part I, Chapter 1 places the outsourcing of warfare by states in historical context. It illuminates the continuities and differences between different earlier forms of non-state provision of coercive services in armed conflicts and the modern phenomenon of PMSCs, and suggests that the attempts to ban "mercenarism" remain largely irrelevant to the modern contractor problem. Chapter 2 tracks the ascendance of military contracting since the end of the Cold War and the state of the industry, submits a typology of services provided by contractors, and delineates the coercive services to be studied. Part II subsequently introduces the norms of IHL and Human Rights threatened by these services, with Chapter 3 addressing combat, and guarding and protection services and Chapter 4 interrogation and detention services. Part III then tackles state responsibility for violations of negative and positive obligations of hiring states. Based on an analysis of the ILC Articles on State Responsibility, Chapter 5 outlines a responsibility gap between the attributability of violations of negative IHL or HRL obligations by a soldier of the national army of a state, as compared to equivalent conduct by military contractor personnel. In a second step, I evaluate the state responsibility provisions of IHL with a view to closing the apparent responsibility gap. Chapters 6 and 7 illustrate the positive obligations of hiring states with respect to contractors exercising coercive services under IHL and HRL even where their conduct is not attributable to the state, and reassess the responsibility gap. By way of conclusion, Chapter 8 provides further reflection of the conceptual aspects raised in the thesis, along with policy recommendations aimed at improving the responsibility and accountability of states relying on PMSCs in their war efforts.