The work of private military and security contractors is extremely controversial from the point of view of international law and of practice. Sometimes there are doubts as to whether some of their activities should be considered legal activities or illegal mercenarism. Like any other entities using force, they can violate human rights as well as international humanitarian law. They provide their services to, amongst others, states and intergovernmental organisations, including the un. This requires a precise definition of the rules under which such contractors operate, both with regard to the law of treaties and the domestic law of the entities using their services. A question also arises as to whether there is any legal limit to their services being used by intergovernmental organisations, i.e. entities deriving their competences from the will of their member states. The work of the un is an interesting example here. The organisation uses such contractors, but on the other hand, it undertakes various activities to eliminate any potential threats in this respect.