Physical and chemical measurements of air pollutants provide a precise measure of pollutant exposure which is frequently used to estimate probable biological impacts. Bioindicators may be classified as either accumulators of the pollutant or reactors to the pollutant. The ultimate selection of a bioindicator depends on the pollutant and the ultimate use of the data. A bioindicator should: (1) provide a readily detectable response to the pollutant; (2) be easy and efficient to use; (3) be readily related to the response of interest; and (4) have a distinctive syndrome not readily confused with other causes. Bioindicators clearly have value in assessing environmental problems, but, at most, they are only indicators of a problem. Corrective or mitigative actions require additional data.