Water use in the United States to the year 2040 is estimated by extending past trends in basic water-use determinants. Those trends are largely encouraging. Over the past 35 years, withdrawals in industry and at thermoelectric plants have steadily dropped per unit of output, and over the past 15 years some irrigated regions have also increased the efficiency of their water use. Further, per-capita domestic withdrawals may have finally peaked. If these trends continue, aggregate withdrawals in the U.S. over the next 40 years will stay below 10% of the 1995 level, despite a 41% expected increase in population. However, not all areas of the U.S. are projected to fare as well. Of the 20 water resource regions in the U.S., withdrawals in seven are projected to increase by from 15% to 30% above 1995 levels. Most of the substantial increases are attributable to domestic and public or thermoelectric use, although the large increases in 3 regions are mainly due to growth in irrigated acreage. The most important and uncertain assumptions necessary to make these projections are those about future irrigated acreage. If irrigated acreage fails to drop in most Western basins, as assumed, withdrawals may be substantially above these projections.