During the first half of the last century a new form of the millennia-old profession of the mercenary emerged. Private military companies (PMCs) came into being, which are performing military actions within the framework of a corporate structure. The end of the Cold War and the collapse of the Eastern Bloc, combined with asymmetric conflicts, led to an enormous boom within the PMC-branch. Modern warfare cannot be imagined without those companies, which are also important actors in zones of conflict or crisis. Several case studies are used to analyze the actions of PMCs in three different regions of the world, with a special focus on human rights protection against those private military actors. Also their actions in service of the United Nations and NGOs are included in the considerations. Furthermore, the position of PMCs and their employees in international law is outlined. Moreover it is analyzed how human rights are protected against actions of PMCs, looking at international treaties regarding mercenarism as well as at other institutions and instruments, like international courts and non-binding approaches, e.g. the Montreux Document. A chapter on further developments of human rights protection against PMCs by contemporary approaches and concepts, like an international convention or voluntarily self-imposed obligations, provides an overview on possible future developments and possibilities regarding human rights obligations of PMCs. At the present, regulation of private military companies is insufficient, at the international, regional and national level, regarding human rights obligations. An effective approach like an international convention can only be achieved by efforts of the civil society.