Quantitative risk assessment is fraught with many uncertainties. The validity of the assumptions underlying the methods employed are often difficult to test or validate. Cancer risk assessment has generally employed either human epidemiological data from relatively high occupational exposures or animal cancer data from lifetime rodent bioassays. Low-dose extrapolation methods are often used with the human epidemiological data to estimate risks at lower environmental exposure levels. Scaling factors are used with animal data to extrapolate from rodents to humans. The problems, controversies, and uncertainties associated with these methodologies has encouraged us to develop the concept and methodology associated with a comparative potency method for cancer risk assessment. This comparative potency method may be particularly appropriate for application to the quantitative cancer risk assessment of combustion emissions.