A database was developed to organize and manage data compiled for a regional assessment of geohydrologic and water-quality conditions in the south San Francisco Bay and Peninsula area in California. Available data provided by local, State, and Federal agencies and private consultants was utilized in the assessment. The database consists of geographicinformation system data layers and related tables and American Standard Code for Information Interchange files. Documentation of the database is necessary to avoid misinterpretation of the data and to make users aware of potential errors and limitations. Most of the data compiled were collected from wells and boreholes (collectively referred to as wells in this report). This point-specific data, including construction, water-level, waterquality, pumping test, and lithologic data, are contained in tables and files that are related to a geographic information system data layer that contains the locations of the wells. There are 1,014 wells in the data layer and the related tables contain 35,845 water-level measurements (from 293 of the wells) and 9,292 water-quality samples (from 394 of the wells). Calculation of hydraulic heads and gradients from the water levels can be affected adversely by errors in the determination of the altitude of land surface at the well. Cation and anion balance computations performed on 396 of the water-quality samples indicate high cation and anion balance errors for 51 (13 percent) of the samples. Well drillers' reports were interpreted for 762 of the wells, and digital representations of the lithology of the formations are contained in files following the American Standard Code for Information Interchange. The usefulness of drillers' descriptions of the formation lithology is affected by the detail and thoroughness of the drillers' descriptions, as well as the knowledge, experience, and vocabulary of the individual who described the drill cuttings. Additional data layers were created that contain political, geohydrologic, and other geographic data. These layers contain features represented by areas and lines rather than discrete points. The layers consist of data representing the thickness of alluvium, surficial geology, physiographic subareas, watershed boundaries, land use, water-supply districts, wastewater treatment districts, and recharge basins. The layers manually digitizing paper maps, acquisition of data already in digital form, or creation of new layers from available layers. The scale of the source data affects the accurate representation of real-world features with the data layer, and, therefore, the scale of the source data must be considered when the data are analyzed and plotted.