In the spring of 2002, the Texas Commission on Environmental Quality determined that perchlorate (C1O4-) was present in ground water from the McMillan and Paul Davis well fields that supply potable water for the city of Midland. Researchers began a large-scale sampling program to determine the source(s) and distribution of perchlorate in the area's ground water. This document summarizes the findings of a large-scale investigation in nine counties carried out from July to December 2002. This program included public water system (PWS) wells and private wells in Andrews, Borden, Dawson, Ector, Gaines, Glasscock, Howard, Martin, and Midland counties, which occupy a total area of 23,960 km2. Water samples were tested for perchlorate and a suite of common ions. From a total of 254 wells sampled in the nine counties, 179 wells (70%) had detectable perchlorate concentrations (>0.5 ppb) and 88 wells (35%) had perchlorate concentrations equal to or above 4 ppb. The highest perchlorate concentration found at a private well was 58.8 ppb in Dawson County, while the highest concentration detected for a well in PWS was 45.6 ppb in city of Midland, Midland County. Perchlorate positively correlated (α < 0.0001) with C1-, F-, Br-, SO42-, Mg2+, and K+ but not with NO2-, NO3-, Na+, or Ca2+. Research to date has identified the most likely sources to be (1) a natural mineralogical impurity; (2) agricultural fertilizers containing perchlorate; (3) in situ generation of perchlorate by electrochemical reactions; or (4) some combination of the three. This study suggests that there may be significant sources other than the traditional industrial processing of perchlorate, and the distribution of perchlorate in ground water is likely more widespread than previously suspected.