Young children and the developing fetus may be more susceptible to effects of environmental toxicants than adults due to differential exposure patterns and developmental immaturities. Biologic markers offer the potential of quantitative dosimeters of biologic dose and/or indices of biologic effect associated with fetal/childhood exposures. They can facilitate evaluation of interindividual variability in response and the magnitude of age-related susceptibilities. Thus far, biologic markers have not been widely used in developmental epidemiology of environmental exposures. Research by our group and others has seen elevations in biologic markers in samples from children and fetal tissue associated with a spectrum of environmental exposures, including tobacco smoke (active and passive), ambient pollution, and dietary contaminants. Studies also suggest that biologic markers can provide powerful dosimeters for investigating reproductive effects. Validation of biologic markers offering the greatest promise for developmental epidemiology is needed.