The 250-square mile study area in southwestern King County, Washington is underlain by sediments as much as 2,200 feet thick, deposited during at least four continental glacial/interglacial periods. Published surficial geologic maps and drillers' lithologic logs from about 700 field-located wells were used to prepare 28 geologic sections; these sections were used to delineate 9 hydrogeologic units--5 aquifers, 3 confining beds, and a basal, undifferentiated unit. Two aquifers in these sediments occur at the land surface. Maps depicting the configuration of the tops of three buried aquifers show the extent and the geometry of those aquifers. Maps showing the thickness of two of the three buried aquifers also were prepared. Potentiometric-surface maps for the major aquifers are based on water levels measured in about 400 wells during April 1987. Hydraulic characteristics of the major aquifers are mapped using more than 1,100 specific-capacity calculations and about 240 hydraulic-conductivity determinations from selected wells. Estimates of the average annual recharge to the ground-water system from precipitation for the entire study area were based on relations determined from modeling selected basins. Discharges from the ground-water system were based on estimates of springflow and diffuse seepage from the bluffs surrounding the uplands, and on the quantity of water withdrawn from high-capacity wells. A total of 242 water samples was collected from 217 wells during two mass samplings and analyzed for the presence of common constituents. Samples also were collected and analyzed for heavy metals, boron, detergents, and volatile organic compounds. These analyses indicated there was no widespread degradation of ground-water quality in southwestern King County.