The quantitation of exposure is one of the most critical elements of the risk assessment process. Contemporary risk assessment models rely on monitoring the target chemical in various environmental media and incorporate a variety of assumptions to estimate this. Recently, biomarkers have been advanced to improve estimates of the delivered dose of a given chemical or mixture of chemicals. A biomarker of exposure may be defined as an indicator of exposure to an environmental pollutant within the exposed organism, which becomes manifest before any adverse effects are evident. Measurement of such a biomarker within a biological system may potentially be used to identify and quantitate directly exposure to hazardous substances. The paper describes a decision model for the selection, evaluation, and validation of a biomarker of exposure. (Copyright (c) 1991 Academic Press, Inc.)