Background: Surface waters, present in every state, are important for many purposes, including drinking water and recreational activity, species' habitats, industrial and economic capacity, and other purposes. Decades of water quality data from the U.S. Geological Survey's National Water Quality Network and numerous EPA, state, and local monitoring systems show that the United States has made significant progress in reducing water pollution levels, but water pollutants continue to harm public health, the economy, and the environment in places across the country. These monitoring systems are unable to provide some of the information needed to better manage risks from water pollution and to achieve the goals of the Clean Water Act, the Safe Drinking Water Act, and other laws. Key Questions: (1) What are the current and potential future capabilities of water quality sensors? (2) What are the benefits and challenges of developing or using these sensors? (3) What policy options, if any, could support various policy goals?