The Basin and Range is an extensive arid to semiarid physiographic province of the western United States that contains several hundred ground-water basins. A complete and detailed hydrogeologic study of this large and complex province would be difficult and extremely costly, so a hydrogeologic classification system is being developed to facilitate the transfer of knowledge gained from a limited number of detailed hydrogeologic characterization studies to relatively unstudied basins. The classification will group together basins that have similar hydrogeologic frameworks through which water moves. In the classification system, each basin is considered to be a control volume with three major hydrogeologic components: soils and streams, basin fill, and consolidated rocks. Basins are classified on the basis of the relative magnitudes of inflows and outflows of water for each of the three hydrogeologic components between basins as well as magnitudes of recharge and evapotranspiration from ground-water or surface-water bodies within the basin. Basins are further classified by the degree of water-resources development and by hydrogeologic features that affect the flow of water through the basin such as surface geology, basin geometry, degree of faulting, and porosity of the major aquifer elements. Geographic information systems are being used to facilitate the classification of basins and the storage of results. The classification system will be used to help prioritize future hydrogeologic characterization studies; for instance, basin categories that lack intensively studied basins would be of higher priority than those categories that have several intensively studied basins.