Water-quality samples were collected from 207 wells in the Santa Ana Basin in the Coastal Range Province of southern California to assess the occurrence and distribution of dissolved constituents in ground water as part of the U.S. Geological Survey's National Water-Quality Assessment (NAWQA) program. These wells were sampled during eight studies from 1999 to 2001 that were designed to sample the used water resource at different scales: (1) three studies characterized water quality at a regional scale; (2) two studies focused on spatial and temporal variations in water quality along flow paths; (3) a land-use study focused on evaluation of water quality in shallow ground water; and (4) two studies assessed aquifer susceptibility to contamination. The Santa Ana Basin is divided into the Coastal Basin, the Inland Basin, and the San Jacinto Basin. The Coastal Basin includes a relatively small unconfined recharge area and a relatively large confined area where ground-water pumping is the primary source of discharge. Land use is almost entirely urban. The Inland Basin is predominantly unconfined and land use is urban and agricultural. The San Jacinto Basin is largely unconfined and land use is mostly agricultural. Water-quality data discussed in this report are compared with U.S. Environmental Protection Agency (EPA) drinking-water standards, both primary and secondary. Most exceedances of maximum contaminant levels (MCLs) occurred in the shallow, coastal monitoring wells that tap ground water not used for water supply. Water from several irrigation wells in the Inland and San Jacinto basins exceeded the 10 mg/L (milligrams per liter) MCL for nitrate. Water from some wells exceeded secondary MCLs for manganese (50 ?g/L [micrograms per liter]) and iron (300 ?g/L) and (or) proposed MCLs for arsenic (10 ?g/L) and uranium (30 ?g/L). Of the 94 production wells sampled for trace elements, 3 irrigation wells in the Coastal Basin produced water that exceeded the secondary MCL for manganese. Water from production wells sampled in all three subbasins exceeded the proposed MCL for radon (300 pCi/L [picocuries per liter]). Pesticides were detected above the laboratory reporting limit (LRL) in 50 percent of the production and monitoring wells sampled in the Santa Ana Basin. Deethylatrazine, simazine, atrazine, tebuthiuron, and prometon were the five most commonly detected pesticides in the current USGS studies. All pesticide concentrations detected in these studies were below MCLs established by the EPA. Volatile organic compounds (VOCs) were detected in 115 wells (56 percent) of the 207 wells sampled. Of the 38 VOCs detected, only 13 were detected in more than five wells. The most commonly detected VOCs, in order of detection frequency, were chloroform; trichloroethlyene, TCE; 1,1,1-trichloroethane, TCA; trichlorofluoromethane, CFC 11; 1,1,2-trichloro-1,2,2-trifluoroethane, CFC 113; tetrachloroethylene, PCE; bromodichloromethane; methyl tert-butyl ether, MTBE; 1,1-dichloroethene, 1-1-DCE; and 1,2- dichloroethene, 1,2-DCE. The only exceedances of EPA MCLs for VOCs occurred in six irrigation wells and in two deep monitoring wells sampled in the Inland Basin.