The Nebraska Cooperative Hydrology Study (COHYST) area covers 29,300 square miles in central and western Nebraska. The main products of COHYST are groundwater-flow models that can be used to analyze the effects of water-management activities proposed under the Three-State Cooperative Agreement among Nebraska, Wyoming, Colorado, and U.S. Fish and Wildlife Service. Hydrostratigraphic units are a critical input to the groundwater-flow models. The hydrostratigraphic units are based on geologic units and their associated hydraulic properties. Ten hydrostratigraphic units, eight within the High Plains aquifer and two beneath it, were based on textural descriptions of the formation characteristics and estimates of hydraulic properties. The geologic units within the High Plains aquifer in Nebraska include Quaternary-age deposits and the Tertiary-age Ogallala Group, Arikaree Group, and fractured Brule Formation. The units beneath the High Plains aquifer include the Tertiary-age White River Group and Undifferentiated Cretaceous-age sediments. These units are meant to be used for regional model purposes and may not be appropriate for specific local characterizations. The hydrostratigraphic units were constructed with a Geographic Information System using available literature, test-hole logs, registered-well logs, observation-well logs, and surficial geologic maps. All available registered well information was put into a database, reviewed, and over 3,800 wells were selected for use in creating the units. These were added to about 2,000 test-hole logs and 340 observation-well logs. A digital coverage of available surficial geologic maps for the area was created. Additional work to refine the hydrostratigraphic units includes new test-hole drilling, shallow seismic surveys, and resistivity surveys. By using this combination of subsurface and surficial geologic information, the regional hydrostratigraphic units were completed, for use in the groundwater-flow models.