This article reviews the literature on the subject of dental unit waterline contamination. It has been expanded from the text of a lecture given at the Scientific Frontiers in Dentistry program sponsored by the National Institute for Dental and Craniofacial Research in Bethesda, Md., in July 1999. The author examines the underlying biological causes of waterline colonization by microorganisms, the evidence of potential health consequences and possible means of improving the quality of dental water. He also describes examples of devices currently marketed to improve and maintain the quality of dental treatment water. Microorganisms colonize dental units and contaminate dental treatment water. While documented instances of related illness are few, water that does not meet potable-water standards is inappropriate for use in dentistry. Exposure to water containing high numbers of bacteria violates basic principles of clinical infection control. Dentists should consider available options for improving the quality of water used in dental treatment.