Relying on the U.S. Geological Survey water use data for the period 1960−2005, this paper summarizes past water use and then projects future water use based on the trends in water use efficiency and major drivers of water use. Water use efficiency has improved in most sectors. Over the past 45 years, withdrawals in industry and at thermoelectric plants have steadily dropped per unit of output. In addition, domestic and public withdrawals per capita, and irrigation withdrawals per unit area in most regions of the west, have recently begun to decrease. If these efficiency trends continue and trends in water use drivers proceed as expected, in the absence of additional climate change the desired withdrawals in the United States over the next 50 years are projected to stay within 3% of the 2005 level despite an expected 51% increase in population. However, including the effects of future climate change substantially increases this projection. The climate-based increase in the projected water use is attributable mainly to increases in agricultural and landscape irrigation in response to rising potential evapotranspiration, and to a much lesser extent to water use in electricity production in response to increased space cooling needs as temperatures rise. The increases in projected withdrawal vary greatly across the 98 basins examined, with some showing decreases and others showing very large increases, and are sensitive to the emission scenario and global climate model employed. The increases were also found to be larger if potential evapotranspiration is estimated using a temperature-based method as opposed to a physically based method accounting for energy, humidity, and wind speed.