The United Nations was formed in 1945 with the major purpose of promoting and ensuring international peace and security. Since then it has formed the major forum for the negotiation and management of major treaty regimes, become central to international law relevant to security, promoted normative and analytical innovation in international security, and – with bodies like the Security Council, the Department of Peacekeeping, the High Commissioner for Refugees, and the Office for the Coordination of Humanitarian Affairs – taken on a direct operational and coordinating role in the governance of international security. This chapter critically surveys the evolving architecture of the UN system as it pertains to security, with a particular focus on the changing role and focus of the Security Council since the end of the Cold War, drawing on issue-mapping research by the authors.