The Connecticut River has a long history of water quality impairment. From the 1800s to the late 1960s, untreated or minimally treated waste discharges from population centers and industries have caused serious water quality problems. Trend analysis of selected water quality data in Connecticut from 1968 to 1998, collected by the U.S. Geological Survey in cooperation with the Connecticut Department of Environmental Protection, shows that water quality has improved, benefiting aquatic plants and animals, recreation, and the esthetics of the river. Many of the trends detected—including downward trends in total phosphorus, total nitrogen, and indicator bacteria, and upward trends in pH and dissolved oxygen—can be attributed primarily to improvements in wastewater treatment following the Federal Clean Water Act of 1972. Some uncertainty remains in evaluating the environmental significance of trends in dissolved oxygen, pH, and other constituents with diurnal fluctuations caused by plant metabolism. Downward trends in sulfate concentrations likely are attributable to reductions in sulfur dioxide emissions mandated by the Clean Air Act of 1970 and amendments made in 1990. Upward trends in chloride concentrations illustrate some effects of increasing urbanization and nonpoint-source pollution. Current (2003) and future water quality challenges for the Connecticut River include reducing nitrogen loads from point and nonpoint sources, reducing bacteria and other contaminant concentrations in urban stormwater runoff, and separating stormwater and sanitary sewers at some locations to prevent combined-sewer overflows.