As global interdependence grows, states often use international organizations to achieve both domestic and foreign policy goals. One way states respond to demands for cooperation is to delegate to international organizations and private actors. In this article, we use new data spanning a century of international environmental law to understand when and why states delegate to international organizations to manage environmental problems. We find that delegation is a persistent phenomenon that facilitates the implementation of states' preferences. However, they make this decision with care: States tend to delegate functions with lower sovereignty costs, such as implementation and monitoring, but rarely delegate rule making and enforcement. We also find that heterogeneous preferences among states increases the likelihood of delegation. Overall, our results suggest that states seek to delegate out of a motivation both to reduce transaction costs and to establish credible commitments.