This chapter examines the development of the United Nations and the changes and challenges that it has faced since it was founded in 1945. It opens with three framing questions: Does the UN succeed in reconciling traditions of great power politics and traditions of universalism? Why has the UN become more involved in matters within states and what are the limits to this involvement? What are the UN's biggest successes and challenges in its efforts to prevent and resolve conflict and to promote sustainable development? The chapter proceeds by providing a brief history of the UN and its principal organs. It also considers the UN's role in the maintenance of international peace and security, and how the UN addresses issues relating to economic and social development. Two case studies are presented: the first is about UN peacekeeping in the Congo and the second is about the 2003 intervention in Iraq.