As debates on reform of global environmental governance intensify, the future of the United Nations Environment Programme (UNEP) has come into acute political focus. Many argue that the organization has faltered in its role as the UN's leading agency for the environment. In this article, I use historical institutional analysis in combination with current international relations and management theory to explain UNEP's creation and evolution. Having described how the creators of UNEP envisioned the nascent organization, I analyze its subsequent performance, identifying the key factors that have shaped its record. I argue that the original vision for UNEP was ambitious but fundamentally pragmatic, and that the organization's mixed performance over the years can be explained by analysis of three factors: its design, leadership, and location. Thus, this article clarifies the record on UNEP's intended function, and lays the foundation for a systematic methodology for evaluating international organizations.