The companies dealing with military and security matters are on the rise, and today they provide services to a very wide client list, which includes states and international organizations (IOs). It is well known that these companies have been deployed in a large number of armed conflicts, and indeed, some of them have become prominent in the sector not only for their military results but also for their abuses of international humanitarian law and human rights. Surprisingly, it is lesser known that the United Nations (UN) has also had recourse to private military and security companies in the context of United Nations military operations with regard to the maintenance of international peace and security.The engagement of private military security companies in UN peace operations entails multiple legal questions. This dissertation is especially intended to shed some light over the extent to which PMSCs deployment in UN peace operations is compatible with international humanitarian law and how the law of institutional responsibility deals with the violations of international humanitarian law committed by such companies when providing services to the UN. For that purpose and to that extent due recourse has been made to the most relevant international law sources on the matter, such as the Geneva Conventions, their Additional Protocols, and other relevant instruments such as the ILC Articles on State and IO responsibility, and the Montreux Document. Jus cogens and international custom, including the practice of international organizations and states, and opinion juris as ascertained by legal scholars and the International Court of Justice, play an essential role in this dissertation, since the United Nations has not become yet a party to any IHL treaty, thus employing an inductive methodology. A comparative approach was adopted in regard to the observations of the most eminent institutions and jurists, and domestic and international courts, including the International Court of Justice and the European Court of Human Rights for the purpose of ascertaining the different rules of attribution of conduct existing in international law. Besides, certain decisions of the latter Court were analysed in order to clarify by analogy whether the application of international humanitarian law and the imputation of acts can function under the same degree of control test.