In response to a congressional request, GAO reviewed the Environmental Protection Agency’s (EPA) Underground Injection Control (UIC) Program, to assess: (1) the extent to which hazardous waste has contaminated underground sources of drinking water; and (2) EPA and state oversight of underground injection of hazardous waste. GAO found that: (1) although there are few confirmed cases of drinking-water contamination, because the contamination is hard to detect, there could be more; (2) monitoring wells have a limited usefulness for large underground areas; (3) neither EPA nor the states require sampling or testing of groundwater immediately above injected waste; (4) EPA did not perform periodic well inspections to ensure compliance with regulations in two states for which it had direct responsibility; (5) 1984 legislation mandated the banning of injection well disposal of hazardous wastes as of August 1988, unless operators could demonstrate that the hazardous waste would not migrate; and (6) EPA believes that most wells currently in operation should pass a demonstration of no migration, meet the more stringent controls, and continue to operate.