Increasingly, and especially since the end of the Cold War, Private Military/Security Companies have been utilised by states in order to fulfil duties relating to warfare. This book aims to examine the legal basis of PMSCs under international law, and analyse the law of state responsibility within the frame of PMSCs. Yet how can international and domestic law best regulate the Private Military industry? In particular, international law has found it difficult to establish a 'hard law', with a binding nature on PMSC and universal applicability. In such regard, this study seeks to indicate the shortcomings of current international law; but also ensure that the topic stays alive for further research by asking questions relating to contemporary issues.