The American people have become increasingly aware of the potential for exposure to toxic material in our environment and of a need for accurate, objective information on the health effects of pollutants. In keeping with that need, the Agency for Toxic Substances and Disease Registry of the U.S. Public Health Service, the Office of Health Research of the U.S. Environmental Protection Agency, the National Institute of Environmental Health Sciences, and the National Institute of Allergy and Infectious Disease requested the Board on Environmental Studies and Toxicology in the National Research Council's Commission on Life Sciences to examine the potential for use of biologic markers in environmental health research. The term "biologic markers" has been used by the National Research Council's Committee on Biologic Markers to refer to indicators of events in biologic systems or samples. It is useful to classify biologic markers into three types—markers of exposure, of effect, and of susceptibility—and to describe the events peculiar to each type.