This volume represents the work of an international group of experts over some years, finally summarized in a meeting at Geneva in 1956. It deals only with health aspects of public water supplies for domestic use. The methods given are carefully selected and are based largely on procedures described in “The Bacteriological Examination of Water Supplies” of the Ministry of Health and the Ministry of Housing and Local Government for England and Wales, and “Standard Methods for the Examination of Water, Sewage, and Industrial Wastes,” 10th edition, of the American Public Health Association, with modifications directed mainly toward simplification. The coliform group is used as the principal indicator organism, and its determination follows the familiar “presumptive,” “confirmed” and “completed” tests. Azide and tellurite methods are given for fecal streptococci and a boiled-milk procedure for Cl. perfringens, and their significance is discussed. Numbers are expressed in MPN based on tables revised by Swaroop. IMViC procedures are given for E. coli.