The results of geochemical and isotopic analyses are discussed for groundwater samples obtained from five mines and several shallow wells in the Sudbury Basin. Mine samples were usually obtained from abandoned diamond drill holes or fractures, which were found from depths of 152–1219 m (500–4000 ft). Two very distinctive water types are recognized in the subsurface environment. Fresh to brackish waters were found from surface to approximately 800 m (2600 ft) in depth. However, below 914 m (3000 ft) very saline or briny waters occur at most of the localities sampled. The geochemistry of the shallow waters shows gradual increases in concentration for most elements; locally effects from the proximity of sulphide ores or drill hole grouting were observed. Isotopic data (2H, 18O, and tritium) indicate that the major component of these waters is less than 30 years of age and represents local meteoric waters. The saline waters have dissolved loads often in excess of 200 g∙L−1 and are dominated by calcium and chloride. The chemistry and isotopic contents of the Sudbury brines are very distinct from highly saline waters or brines of sedimentary or geothermal origin. The brines appear to represent very old, stagnant "groundwaters," which may have undergone prolonged chemical and isotopic alteration since their original emplacement. At the present a variety of possible origins for the deep Sudbury waters can be postulated, but most models for brine formation found in the literature are unsatisfactory to explain the origin and genesis of these deep Canadian Shield brines.