Introduction. Collection and fixation of water. Dissolved oxygen. Oxygen demand and oxygen consumed. Hydrogen-iion concentratio, pH. Alkalinity and acidity. Suspended matter. Water hardness. Calcium and magnesium. Iron determinations. Magnanese; Copper; Zinc; Sulfates; Sulfides, sulfites, and other non-sulfate sulfur compounds; Phosphate determinations; Fluorides; Chlorides; Chlorine; Acetylene. Speical determinations. Exploratory tests. Fluorescence. Effluents and trade wastes. Evaluation of pollution hazards. Acknowledgments. Literature cited. This manual on water quality has been designed to present a system for the determination and evaluation of those conditions and substances of primary interest to the aquatic biologist, which occur in natural and polluted waters. Many of the procedures are applicable, however, to various problems in sanitary engineering and in trade waste disposal. The methods chosen have been used by the authors of the Water Quality Laboratories of the United States Fish and Wildlife Service and in actual field operations, in most cases, over a period of years. It has been possible in general, therefore, to select analytical sequences for which the technical limitations are rather well established. Where it seemed desirable, a choice of procedures, including both routine and research methods, is offered.