The potential of biologic markers to provide more timely and precise risk assessments for environmental carcinogens is viewed against the current state-of-the-art in biological monitoring/molecular epidemiology. Biologic markers such as carcinogen-DNA adducts and oncogene activation are currently considered valid qualitative indicators of potential risk, but for most chemical exposures research is needed to establish their validity as quantitative predictors of cancer risk. Biologic markers have, however, already provided valuable insights into the magnitude of interindividual variation in response to carcinogenic exposures, with major implications for risk assessment.