“OECD countries have struggled to adequately address diffuse water pollution. It is much easier to regulate large, point source industrial and municipal polluters than engage with a large number of farmers and other land-users where variable factors like climate, soil and politics come into play. But the cumulative effects of diffuse water pollution can be devastating for human well-being and ecosystem health. Ultimately, they can undermine sustainable economic growth. Many countries are trying innovative policy responses with some measure of success. However, these approaches need to be replicated, adapted and massively scaled-up if they are to have an effect.” After decades of regulation and investment to reduce point source water pollution, OECD countries still face water quality challenges (e.g. eutrophication) from diffuse agricultural and urban sources of pollution, that is disperse pollution from surface runoff, soil filtration and atmospheric deposition. The relative lack of progress reflects the complexities of controlling multiple pollutants from multiple sources, their high spatial and temporal variability, associated transactions costs, and limited political acceptability of regulatory measures. This report outlines the water quality challenges facing OECD countries today, presents a range of policy instruments and innovative case studies of diffuse pollution control, and concludes with an integrated policy framework to tackle diffuse water pollution. An optimal approach will likely entail a mix of policy interventions reflecting the basic OECD principles of water quality management – pollution prevention, treatment at source, the polluter pays and beneficiary pays principles, equity, and policy coherence.