Outsourcing military and security services to the private sector is an emerging trend under international law. The shift to using private military and security companies (pmscs) in countries such as Iraq and Afghanistan has brought attention to the role that these companies may play in fulfilling functions that are normally monopolised by States or international organisations. The reliance of the UN on pmscs has increased considerably in recent years, leading to the question of the legality of their use in various UN operations. This paper focuses on two main aspects of the UN’s use of such companies; (i) The engagement of pmscs in peacekeeping operations, either when hired by the UN directly or when hired by a State and subsequently seconded to the UN; and (ii) The participation of pmscs in peace enforcement measures adopted by the UN Security Council according to Chapter vii of the UN Charter. This paper argues that the use of pmscs in peacekeeping operations is lawful under international law, while their use in peace enforcement operations is not.