The use of biomarkers was reviewed and discussed, with particular attention given to their validation, their use in environmental medicine, and the ethical and legal issues associated with their use. Biomarkers were classified into markers of exposure, effect and susceptibility. Biologic markers of exposure included exogenous substances or their metabolites, or the reaction product formed from an interaction between a substance and a cell or molecule from a target organism. Biologic markers of effect included all quantifiable alterations in an organism that can be identified as related to disease or impairment. Biologic markers of susceptibility were indicators of an organism's limits in responding to xenobiotic substance exposure. Applications of biomarkers included their use as indicators for toxic exposure assessment and as determinants of mechanisms of toxicity. Biologic markers have been used in the risk assessment of hazardous substances, and to identify subpopulations of particular susceptibility. Ethical and legal issues related to the research on, and use of biologic markers included subject confidentiality and privacy, as well as qualification for employment or insurance coverage.