The toxicity of water pollutants to aquatic animals is reviewed, with particular emphasis on methods for measuring lethal toxicity, factors influencing toxicity, the measurement of chronic and sublethal toxicity, and the role of toxicological data in formulating water quality standards. Methods for measuring lethal toxicity are well established and have been applied to a wide range of fish and invertebrate species. Their applications and limitations are discussed. The measurement of sublethal toxicity employs very diverse techniques ranging from biochemical to the use of experimental ecosystems. Profitable techniques are those which possess one or more of the characteristics: sensitivity, specificity, ecological relevance. A large data base now exists on the toxicity of pollutants to aquatic species, singly and in combination, and on the effects of environmental conditions, and interspecific and intraspecific biotic factors. Toxicological data and information from field studies are complementary, and their use in formulating water quality standards for the preservation of aquatic life is discussed.