The growing number of international organisations involved in ‘state-building’ and the scope of authority they exercise raises a number of important questions under international law - as to the status of UN-administered territories, the nature o f UN authority, its legal basis in the UN Charter, and its limitations, for example. By looking at the ways through which international authority carries out internationalisation projects, the thesis aims to explain how legal instruments were designed in order to respond to a spatio-temporal need of the international community. It adopts a broad topological style which interrogates where and how to ‘locate’ the background assumptions guiding the idea of international fiduciary administration, in legal and philosophical space. By supplying complementary theoretical frameworks to account for instances of suspended sovereignty, the thesis presents a synoptic vision of the notion of internationalisation of territory and the extent to which multilateral institution-building missions share features with, and can be distinguished from, projects undertaken under the Mandate- and Trusteeship systems. It utilises institutions of both private law (agency, trusteeship, servitude) and public law (wardship, the status of organs) to analyse the dual nature of international administrations. Firstly, an international administration represents a non-state territorial entity on the international plane as agent ex lege. Second, the thesis investigates the organic framework through which an ancillary organ of the UN dispenses temporary political authority in order to carry out the functions, and meet the needs, of the international community. The constructive approach to international legal personality solidifies the argument that a non-state territorial entity administered by the international community may base its claim towards partial personality on a legal argument. The doctorate follows a trajectory that outlines the phenomenon of 'dual functionality' throughout colonialism, trusteeship administration, military occupation and territorial administration on the basis of an international mandate. The notion of the fiduciary bond underpin all examples which illustrate that the more 'international’ the mandate of a territorial administration, the more pronounced its assumption of agency and pursuit of the ‘territorial’ interest. Both frames are applied to the UN Council for Namibia and to the United Nations Interim Administration Mission in Kosovo (UNMIK). The tensions resulting from the simultaneous performance, by the same actor, of the functions of territorial agent and international organ accompany the investigation into the status of Kosovo in public international law. Moreover, the thesis examines certain inherent shortcomings to an ‘open-ended’ institutionbuilding operation where the future status of the entity in statu nascendi remains undecided. Focusing on the internal political and legal order of an internationalised territory, the thesis notes that the rule of an international administration is subject to an ‘anomalous’ legitimacy cycle. The fundamental indeterminacy of law, gaps in statutory instruments and in human rights protection further expose the frailty of transitional administrations.