The findings and recommendations of a study which was conducted to explore the potentially harmful effects that impurities in water may have on the health of those drinking it are explored. The study, whose purpose, scope, and duration were outlined in the Safe Drinking Water Act of 1974, was carried out by the Committee on Safe Drinking Water of the National Research Council. The general approach, principles, and criteria adopted in the study are outlined following a brief historical note on the search for pure water. The procedure involved a review of the scientific literature. The discussion which follows deals with safety and risk assessment. Attention is given to the scientific methods and criteria used for evaluating the long-term effects on and hazards to humans. The principal microbiological contaminants of drinking water in the United States -- bacteria, viruses, and pathogenic protozoa -- are then discussed. Subsequent material focuses on solid particles in suspension, inorganic solutes, organic solutes, and radioactivity in drinking water. Tables, figures, and appendixes containing an executive summary and material on legislation accompany the text.