Biomarkers used in epidemiology were discussed with respect to scientific issues and ethical concerns. Currently used biomarkers were characterized by: the ability to detect xenobiotics at concentrations at the cellular and molecular level; the ability to detect earlier biologic changes presumptive of disease or disease risk; and to identify a detailed continuum of events between an exposure and resultant disease. Biomarkers must be valid, reliable, and practical if they are to enhance cancer epidemiology. Pilot studies should be conducted prior to the primary study when these characteristics have not been demonstrated previously. In order for the marker selected to be a useful one it is important that there be interdisciplinary communication and collaboration. This will also improve the interpretation of the findings, making them more appropriate to the marker under study. To validate a marker for use in a population, attention must be paid to issues of background prevalence, sample size, natural history, persistence, variability, confounding factors, and predictive value. Practical features such as subject preparation, access to and storage of specimens, and costs must be clarified. The use of biologic markers in epidemiologic studies will ultimately depend on how well these markers increase the ability to reduce misclassification, provide for better understanding of exposure and disease associations, and increase the number of opportunities to prevent the disease from occurring. Implications ranged from uncertainty about the marker's meaning to societal responses to groups expressing abnormal marker frequencies.