This review presents laboratory data and their interpretation with regard to effects of pollutants on marine and freshwater unicellular algae. Stimulation and inhibition of growth are considered to be equally undesirable. It is suggested that a search for new species for use in toxicity tests be made because the widely-used green alga, Selenastrum capricornutum, may not be one of the most sensitive freshwater species. Several algal species should be used to test each toxicant because there is not a single most-sensitive species and the conditions under which species are tested affects response. The batch method is the most commonly used exposure system for defining relative effects of pollutants, but mixed-species continuous culture presents a more realistic approach to estimation of effects in natural systems. Algae are more sensitive to industrial and energy-process wastes than animals. They respond to growth stimulants and growth inhibitors, and the stimulation response occurs at concentrations much lower than those that inhibit growth.