The Lompoc area is a structural basin that occupies the westernmost region of the Santa Ynez River basin, Santa Barbara County, California. The study area includes the Lompoc plain, upland, and terrace. The nearly flat alluvial deposits of the Lompoc plain and temperate climate make conditions favorable for large scale irrigation farming. In addition, the demand for water in the Lompoc area for domestic and municipal uses has significantly increased since the 1960s. As the demand for ground water has increased, the quality of ground water has deteriorated in several parts of the Lompoc plain. There is concern that continued deterioration of ground-water quality will result in the ground water becoming unusable for most purposes. Before regulatory agencies can develop management plans on ground-water availability and quality, however, there needs to be a better understanding of the geohydrology of the Lompoc area. The objectives of this study were to describe the geohydrology of the Lompoc area and evaluate ground-water flow using numerical simulation. Two principal aquifers comprise the ground-water system in the Lompoc area. The upper aquifer contains the shallow, middle, and main zones. The main zone is the primary source of water to the Lompoc plain. The lower aquifer also is present beneath the Lompoc plain and extends to the Lompoc upland and terrace. In 1988, ground-water movement in the shallow and main zones of the upper aquifer generally was from east to west, and in the lower aquifer from the upland and terrace to the plain. During the irrigation season, ground water flowed toward the main zone from both overlying and underlying water-bearing deposits. Relatively large hydraulic-head differences occurred between the shallow zone and underlying water-bearing deposits in 1988 beneath the central and western plains. A three-dimensional finite-difference model, MODFLOW, was used to simulate and evaluate ground-water flow in the plain, upland, and terrace from 1941 to 1988. The aquifer system was simulated as four horizontal layers. Model-calibrated average annual recharge and discharge were estimated to be 24,430 acre-feet per year and 26,660 acre-feet per year, respectively. The amount of water recharging the main zone from the lower aquifer averaged about 8,000 acre-feet per year for the 48-year simulation period. Model-calibrated long-term storage depletion indicates that the Lompoc area aquifer system has been overdrafted. Estimated perennial supply for the period 1941-88 (the average annual difference between pumpage and storage) equaled 20,640 acre-feet. In 1988, pumpage from the aquifer system exceeded the perennial supply by 10,000 acre-feet. A limiting factor to the perennial supply in the Lompoc area in 1988 was ground-water quality deterioration associated with municipal and irrigation pumpage in the eastern and western plains, respectively. If hydraulic head in the main zone near the coast remains at 1988 levels, seawater intrusion will not be a limiting factor to the perennial supply.