Zooplankters have potential value as assessors of trophic conditions. They respond quickly to environmental change and may be effective indicators of subtle alterations in water quality. Since most species are widely distributed in diverse environments, those with greatest value are ones limited to extremes of trophic lake types (i.e., oligotrophy, eutrophy, and dystrophy). In the wide range of ill-defined intermediate lake types, quantitative data on zooplankton community composition offers more potential than qualitative information on the presence of absence of certain species. The ratio of calanoid copepods to other major groups of zooplankton appears to be of value in identifying relative differences in trophic conditions. Multivariate analyses based on distribution and abundance of rotifer and crustacean species have proved useful in delineating major water masses of different trophic conditions in large lake systems. But caution must be exercised in establishing one-to-one causal relationships between zooplankton composition and trophic conditions since other factors, especially toxic pollutants and size-selective predation, may exert considerable influence on changes in the community composition.