Bainbridge Island is underlain by as much as 1,600 feet of unconsolidated deposits of Quaternary age. Most domestic groundwater supplies are taken from two relatively coarse-grained geohydrologic units composed largely of glacial sand and gravel. Neither of these two aquifers, generally separated by a finer-grained semiconfining unit, is laterally continuous across the entire island. Groundwater withdrawal on the island in 1984 was estimated to be 1,235 acre-feet. Of this amount, 60 percent (740 acre-feet) was withdrawn for public-supply purposes, 37 percent (460 acre-feet) for domestic purposes, and about 3 percent (35 acre-feet) for industrial purposes. The chemical quality of ground water on Bainbridge Island generally is suitable for most uses, and most samples were within State drinking-water standards. However, 3 of 48 samples exceeded the criterion for iron and 19 exceeded the criterion for manganese. This is not considered to be a major water-quality problem and is probably due to natural causes. Chloride concentrations in ground water were small in both sampling periods, April and September 1985, and changed little seasonally. These observations indicate that seawater intrusion currently is not a problem on Bainbridge Island. The data now available are adequate to permit an assessment of the groundwater resources of the island, but only in a qualitative manner and only for the uppermost part of the thick unconsolidated deposits. The data are not adequate, however, to permit an assessment of the effects of additional groundwater development. This would require the construction of a mathematical model of the groundwater system, which in turn would necessitate extensive deep exploratory drilling.