Diffuse, or non-point source pollution, derives from a vast array of activities which involve no single distinct source thus making it difficult to manage with regulations. Efforts to do so have been gaining considerable momentum both internationally and in New Zealand. This research examines how approaches to regulate diffuse pollution differ between New Zealand and the United States, and what challenges these differences present for water quality policy implementation. It uses a comparative case study approach and focuses on the catchments of Te Waihora and Chesapeake Bay. Drawing on a conceptual framework that focuses on factors which influence policy implementation, this study highlights that although setting quantitative limits may appear ideal in theory, the complexity of diffuse pollution and the capacity of governments to regulate it make the implementation of resource limits very challenging.