The Trusteeship Council, a UN principal organ, is responsible for the Trusteeship System, an institutionalized form of colonial administration broadly following the League of Nations Mandates arrangements. This system came to be repudiated, alongside other forms of colonialism, by the external self-determination entitlement that emerged in international law after the creation of the UN in 1945. The present chapter details the concept of ‘trust’ in international policy; the central features of the Mandates and Trusteeship arrangements; the territories covered; the objectives and duration of the arrangements; the structure of administration and supervision; the historical controversy over South West Africa/Namibia; the self-determination entitlement; the revival of trusteeship; reform proposals; the new Peacebuilding Commission; and the continuing use of the Trusteeship Council chamber.