The objective of this master’s thesis is to understand how reforms in the United Nations (UN) development system often fail or has its original course and impact altered. The UN has been heavily exposed to criticism in the past decades and the multilateral organization is considered to suffer from paralysis and inefficiency. There are multiple causes for these conditions, and the conflicting interests of the member-states are often to blame, as consensus on reform content and procedures become increasingly difficult to agree on. The polarization among donors and recipients regarding the future aid structure influence the United Nations in a potentially negative manner. The criticism of the UN is also centred on the internal structure, where the various agencies, funds and programmes are duplicating each other and mandates are proliferated. This results in an incoherent development effort, where the resources and legitimacy are utilized to an unsatisfactory level. This has propelled a clear message from member-states and observers, demanding that the United Nations need to implement reforms sooner rather than later – a task that has proved difficult in the past, according to researchers and observers. The latest reform initiative, initiated by Kofi Annan, was launched in 2006 and aims at creating a more coherent and harmonized UN on a global, regional and national level. The rhetoric of the reform is ‘One UN – Deliver as One’, indicating that the UN bodies in the developing countries, for example, must coordinate there various programmes and strategies into One Plan with One Budget under One Leader, have One Organizational structure and move in to One UN House where appropriate. This master’s thesis focuses on the One UN reform and Vietnam, one of eight pilot countries, has been selected as a case study to describe and understand how the development agencies of the United Nations decides and implements the reform. The UN Vietnam has been chosen, because it is the most advanced One UN pilot countries, as well as having a progressive Government and Donor Community wishing to harmonize the performance of the UN. Hypotheses constructed from organizational theory regarding leadership, support and authority alongside concepts such as identity and communication form the baseline for the investigation of the internal relations in the UN agencies. The implications of member-states’ actions and strategies regarding the implementation of the One UN reform are analysed in relation to selected hypothesis from international relations theory where the key concepts are power and interests. The investigation is empirically substantiated through qualitative interviews with leading officials from the United Nations Country Team in Vietnam as well a representative from the donor community. The conclusions of this master’s thesis reflect that the often polarized member-countries and the UN agencies must seek a consensus and employ a stronger will to change if the One UN reform is to be implemented successfully – a difficult quest, given the conflict of interests and struggle for power in the world of international politics. The distance from UN Headquarters to the country level also proves vast – making successful implementation of the One UN reform vulnerable and too dependable on personalities, as the Vietnam case proved. The UN Vietnam case also reflects that reform must be double-sided – reform initiation from the headquarters level is critical, yet increased support and authority to the country level is essential in creating a coherent, efficient and relevant UN development system.