Drinking water for approximately one sixth of US households is obtained from private wells. These wells can become contaminated by pollutant chemicals or pathogenic organisms, leading to significant illness. Although the US Environmental Protection Agency and all states offer guidance for construction, maintenance, and testing of private wells, there is little regulation, and with few exceptions, well owners are responsible for their own wells. Children may also drink well water at child care or when traveling. Illness resulting from children's ingestion of contaminated water can be severe. This report reviews relevant aspects of groundwater and wells; describes the common chemical and microbiologic contaminants; gives an algorithm with recommendations for inspection, testing, and remediation for wells providing drinking water for children; reviews the definitions and uses of various bottled waters; provides current estimates of costs for well testing; and provides federal, national, state, and, where appropriate, tribal contacts for more information. Pediatrics 2009;123:e1123-e1137 BACKGROUND Approximately 15% to 20% of households in the United States obtain their water from private wells. 1 Public drinking water systems are regulated by the US Environmental Protection Agency (EPA), with national drinking water regulations providing the legally enforceable standards. Unlike municipal water supplies and some community wells, private wells are not subject to federal regulations and are minimally regulated by states. States sometimes require that a well be dug or drilled by a certified contractor and that the water from the well be tested at least once for nitrate and coliform bacteria. After that, the owner of the well is not required to inspect the well or test the water; only New Jersey requires testing at the time of resale. The states, the Navajo Nation, and the EPA offer suggested inspection and testing schedules (Appendix).Well water is not sterile, nor does it need to be, but it should be free of fecal contamination; such contamination is usually detected by coliform bacteria counts. In Iowa wells in the 1990s, 27% had coliforms. 2 Rigorous data are not available to compare the frequency of illness between children drinking well water versus municipal water. In a Canadian study of 235 rural households using well water, the odds of a child younger than 10 years having an episode of gastrointestinal illness, given the presence of at least 5 colony-forming units of Escherichia coli in the water, was 4.2 (95% confidence interval: 1.1-16.2) times higher than that for adults older than 50 years. 3 However, the risk as compared with the child drinking uncontaminated water was not studied. In a clinical trial of reverse-osmosis water filters, which should remove all infectious agents, in families drinking municipal water meeting bacteriologic standards, approximately 30% of acute gastrointestinal illnesses were prevented by the filters, with no difference according to age group. This study showed that even...