The measurement of chlorophyll a concentrations in periphyton and plankton is now widely used to estimate algal standing crops and photosynthetic rates, the trophic status of surface waters, and the effects of effluents. The relative abundance of chlorophyll a, b and c is characteristic of the various major groups of algae and provides information on the taxonomic composition of the algal community. Measurements of many other properties of periphyton and phytoplankton communities related to standing crop, community structure, and function have been correlated with parameters such as temperature, total phosphorus, cell number, cell volume, cell surface area, carbon content, primary production, biomass, community structure, and diversity. The use of chlorophyll to estimate the biomass and productivity of periphyton began in the 1950s, and a very extensive literature now exists on the subject. Examples of the use of periphyton chlorophyll measurements in water quality monitoring can be found in the chlorophyll-biomass relationship, now called the Autotrophic Index. Because of its potentially wide application in measurements of the effects of pollution on periphyton communities, it has been incorporated into the Model State Water Monitoring Program and Basic Water Monitoring Program of the U.S. Environmental Protection Agency. (Copyright (c) 1986 by the Ohio State University.)