Biological markers provide the opportunity to rapidly advance the field of risk assessment for both workers and the general population. Markers of exposure allow much more accurate assessment of exposure than currently possible using mathematical models of dispersion and uptake. Markers of effect can lead to the prediction of disease sufficiently early to avoid adverse outcomes in workers or the general population. Linkage of predictors of effect with markers of exposure can provide dose-response information of unique relevance to the problems of extrapolating from high to low dose and from animals to humans. Biological markers of susceptibility allow definition of the populations for whom risk characterization is particularly of importance. Development of biological markers suitable for risk assessment requires careful validation and faces impediments due to the overreliance of regulators on mathematical models, which inhibits the development of new bioscience, and due to misplaced concerns about ethical experimental exposures of humans to environmental levels of pollutants. The path to developing biological markers of exposure, effect, or susceptibility of value in risk assessment is primarily through improving basic biomedical understanding.