Expanding development and the prolonged drought from 1999 to 2002 drew attention to the quantity and sustainability of the groundwater resources in Warren County, Virginia. The groundwater flow systems of the county are complex and are controlled by the extremely folded and faulted geology that underlies the county. A study was conducted between May 2002 and October 2008 by the U.S. Geological Survey, in cooperation with Warren County, Virginia, to describe the hydrogeology of the metamorphic and siliciclastic fractured-rock aquifers and groundwater availability in the county and to establish a long-term water monitoring network. The study area encompasses approximately 170 square miles and includes the metamorphic rocks of the Blue Ridge Physiographic Province and siliciclastic rocks of the Great Valley section of the Valley and Ridge Physiographic Province. Well depths tend to be shallowest in the siliciclastic rock unit (predominantly in the Martinsburg Formation) where 75 percent of the wells are less than 200 feet deep. Median depths to bedrock are generally less than 40 feet across the county and vary in response to the presence of surficial deposits, faults, siliciclastic rock type, and topographic setting. Water-bearing zones are generally within 200 feet of land surface; median depths, however, are slightly deeper for the hydrogeologic units of the Blue Ridge Province than for those of the Great Valley section of the county. Median well yields for the different rock units generally range from 10 to 20 gallons per minute. High-yielding wells tend to cluster along faults, along the eastern contact of the Martinsburg Formation, and within potential lineament zones. Specific capacity is relatively low and ranges from 0.003 to 1.43 gallons per minute per foot with median values from 0.12 to 0.24 gallon per minute per foot. Transmissivity values derived from specific capacity data range over four orders of magnitude from 0.6 to 380 feet squared per day. Estimates of effective groundwater recharge from 2001 to 2007 ranged from 2.4 to 29.4 inches per year in the Gooney Run, Manassas Run, and Crooked Run Basins, with averages of 15.3, 14.2, and 5.3 inches per year, respectively. Base flow accounted for between 57 and 86 percent of mean streamflow in the Gooney Run and Manassas Run Basins and averaged about 70 percent in these Blue Ridge Province basins. In the siliciclastic rock-dominated Crooked Run Basin of the Great Valley, base flow accounted for between 33 and 65 percent of mean streamflow and averaged about 54 percent. The high base-flow index values (percentage of streamflow from base flow) in these basins indicate that groundwater is the dominant source of streamflow during wet and drought conditions. About 50 percent of the precipitation that fell on the Blue Ridge basins from 2001 to 2007 was removed by evapotranspiration, and between 33 and 36 percent of the precipitation reached the water table as effective recharge. Nearly 76 percent of the precipitation was removed by evapotranspiration in the Crooked Run Basin, and effective recharge averaged about 12 percent of precipitation between 2001 and 2007. Average values of runoff in all three basins were less than 15 percent of precipitation. Groundwater flow systems in the county are extremely vulnerable to current climatic conditions. Successive years of below-average effective recharge cause declines in water levels, spring discharges, and streamflows. However, these systems can recover quickly because effective recharge increases with increasing precipitation. Lack of precipitation, especially snow, during the critical recharge period (January-April) can have an effect on the amount of recharge to the groundwater system and eventual stream base flow. Estimated values of annual mean base flow have approached and have been below the average regression-derived recharge rates during a period classified as having above-average precipitation. This relation is indicative