Effective prevention of cancer requires sensitive early warning systems to identify groups, and ultimately individuals, who are at high risk of developing cancer and to accurately estimate the magnitude of their risk. Incorporated with molecular epidemiologic studies, biologic markers have the potential to provide quantitative human data on the biologically effective dose of carcinogens, resultant molecular effects, and genetic/acquired factors that modulate these effects. Clearly, this information is directly relevant to risk identification and to risk quantification.