In the 1980s it was determined that existing ambient and compliance-monitoring data could not satisfactorily evaluate the results of hundreds of billions of dollars spent for water-pollution abatement in the United States. At the request of the US Congress, a new programme, the National Water-Quality Assessment, was designed and implemented by government agency, the US Geological Survey (USGS). The Assessment has reported status and trends in surface- and ground-water quality at national, regional, and local scales since 1991. The legislative basis for US monitoring and data-sharing policies are identified as well as the successive phases of the design and implementation of the USGS Assessment. Application to the Danube Basin is suggested. Much of the water-quality monitoring conducted in the United States is designed to comply with Federal and State laws mandated primarily by the Clean Water Act of 1987 and the Safe Drinking Water Act of 1986. Monitoring programs generally focus on rivers upstream and downstream of point-source discharges and at water-supply intakes. Few data are available for aquifer systems, and chemical analyses are often limited to those constituents required by law. In most cases, the majority of the available chemical and streamflow data have provided the information necessary to meet the objectives of the compliance-monitoring programs, but do not necessarily provide the information requires for basin-wide assessments of the water quality at the local, regional, or national scale.