More than 100 million Americans depend on groundwater as a source of drinking water. Three quarters of U.S. cities get their water supplies totally or in part from groundwater and one-third of the largest cities rely on groundwater for at least part of their potable water supply. A number of recent studies have documented the actual or potential contamination of the nation's groundwater on a wide scale. Some of this contamination is man made but much of it is of natural origin. In either case some form of treatment is required before contaminated groundwater can be used for water supply. USEPA's Drinking Water Research Division has been conducting bench, pilot and field studies on technologies that effectively treat groundwater. The paper reports some of the results from these studies.