Severe ambient water pollution is common in many developing countries. A broad array of regulatory and other policy instruments can be used to improve water quality. However, some approaches have been studied more than others, and there are many additional challenges that are specific to the developing country setting. This article describes a range of prescriptive and market-based regulations, voluntary programs, and other policy instruments to control water pollution and also reviews the empirical evidence on the effectiveness of these approaches in practice, with a focus on developing countries. We also examine additional challenges for implementing and assessing such policies in developing countries, including data availability and quality issues, insufficient monitoring and enforcement, rent-seeking in regulatory systems, and jurisdictional spillovers where regulation is decentralized. Finally, we highlight important gaps in the published empirical research in this area.