This article seeks to explain why the World Bank's environmental performance is so uneven despite numerous reform efforts. I argue that a principal-agent model offers a potentially powerful tool for analyzing gaps between the mandates and performance of international organizations (IOs) such as the World Bank. The model is particularly useful when it is calibrated to recognize problems of antinomic delegation and the dual role an IO may have as both agent and principal. Antinomic delegation occurs when states ask IOs to take on complex tasks that are difficult to institutionalize. Recognizing that many IOs may be principal and agent at different stages of the policy process reveals more opportunities for agency slack that are not well addressed by the IO literature. This article presents these modifications to the principal-agent model and applies the model to the case of the World Bank. The case study demonstrates that the nature of the tasks being delegated and the incentives shaping both sides of the principal-agent relationship are key sources of disconnect between the institution's stated goals and its performance.