The problems encountered in trans-species extrapolation of the results of chemical carcinogenesis experiments to man are considered from the viewpoints of experimental techniques for establishing chemicals as carcinogens and the effect of modifying factors therein. Extrapolations from the effects of high doses of carcinogens in experimental animals to low doses in man present problems which are beyond the reach of present epidemiological and experimental methodology. The author prefers a pragmatic approach to the latter problems rather than the use of largely unsupported mathematical methodology.