At the moment it is not exactly clear what the rights and obligations of states are regarding Private Military Companies under international law. Nevertheless the reliance of states on their services has increased. Especially during hostilities the possibility of an internationally wrongful act increases, which can trigger State Responsibility. This book determines at what precise point State Responsibility is triggered with regard to the Articles on State Responsibility and the due diligence principle, so that states can be held responsible for the acts of Private Military Companies. This book also offers some elements for regulating Private Military Companies on a global level and discusses the contribution of new initiatives, such as the newly finalized Montreux Document and the global framework of the International Peace Institute. This book is useful for academic professionals, government officials, private military companies, non-governmental organizations and anyone who has a special interest in this topic.