The study of aquatic organisms as they have been related to water supply and water pollution problems since 1850 is detailed. Significant contributions have been made that relate plankton, benthos, periphyton, and fish to the definition of organic, toxic, thermal, and silt pollution. Generally it is not realistic to isolate a particular genus or even a species of aquatic organism to indicate the presence or absence of pollutional wastes in water. It is the study of the total aquatic biota that tells one most about water conditions. Nevertheless, something equated with the magnitude of the problem that may be termed 'reality' often dictates the type of study and the kinds and numbers of samples collected. Serious thought should be given in the reporting of data to ensure that the final report is matched to the needs of the study and provides answers to questions responsible for the instigation of the study. (Author)