The current study (Volume 4) identifies chemical and biological contaminants associated with drinking water distribution systems and the health implications of deficiencies in those systems. Topics covered include chemical effects and water quality, water treatment chemicals, pipes and linings, biological effects and water quality and implications for human health. It also contains an evaluation of information on the toxicity of selected inorganic and organic contaminants. Some of them are reviewed for the first time in this report; others were reviewed in earlier volumes of this series. For the latter, discussions include only information that has become available since the earlier reports.