The California State Water Resources Control Board (State Water Board) initiated the Oil and Gas Regional Monitoring Program (RMP) to assess effects of oil and gas development on groundwater designated for any beneficial use. The U.S. Geological Survey (USGS) is the technical lead in conducting the RMP through the California Oil, Gas, and Groundwater (COGG) Program, working in cooperation with the State Water Board, and in partnership with other State and local agencies. The USGS collected and analyzed groundwater, surface water, and associated quality control (QC) samples in the Elk Hills study area during June 2017–April 2018. The Elk Hills study area includes the Elk Hills and North Coles Levee Oil Fields and a five-kilometer buffer zone surrounding the administrative boundaries of the two oil fields in Kern County, California. Water samples were collected from twenty-six monitoring wells, four irrigation wells, two public supply wells, and one managed recharge pond. Samples were collected using established groundwater data-collection protocols and procedures. Samples were analyzed for water-quality indicators, major and minor ions, nutrients, trace elements, volatile organic compounds (VOCs), naturally occurring radioactive material, geochemical and age-dating tracers, dissolved organic carbon (DOC) characteristics, dissolved standard and hydrocarbon gases (methane through hexane), and dissolved noble and atmospheric gases. Over 200 constituents and water-quality indicators were measured. Quality-control samples collected as part of this study included replicates, field blanks, equipment blanks, source-solution blanks, and matrix spikes.