The United Nations Secretariat is one of the six principal organs of the United Nations (UN), The secretariat is the UN's executive arm. The secretariat has an important role in setting the agenda for the deliberative and decision-making bodies of the UN, and the implementation of the decision of these bodies. The secretary-general, who is appointed by the General Assembly, is the head of the secretariat.

The United Nations System consists of the United Nations' six principal bodies, the Specialized Agencies and related organizations. The UN System includes subsidiary bodies such as the separately administered funds and programmes, research and training institutes, and other subsidiary entities. Some of these organizations predate the founding of the United Nations in 1945 and were inherited after the dissolution of the League of Nations.