Nutrient pollution, defined as excess amounts of nitrogen and phosphorus in aquatic systems, is one of the leading causes of water quality impairment in the United States. This report compiles current information regarding the costs of nutrient pollution. Such costs may be of two broad types. Some costs are associated with reducing nutrient pollution at its sources. Other costs are associated with the impacts of nutrient pollution in the environment. The latter category of costs is referred to as “external costs” or “externalities,” because they are “external” to the owners of the farms, businesses, or facilities that generate them.