The present workshop had its orlglns in discussions among Alexander Hollaender, Oddvar Nygaard, Donald Borg, Richard Setlow and Victor Bond on the need for a symposium that would deal with a broad spectrum of pressing subjects related to the physics, chemistry, and biological actions of ionizing radiations, and the theoretical and practical problems of risk assessment. It soon became apparent that the spectrum of subjects considered was too broad for the desired depth of coverage; in fact, it seemed un­ likely that the conference participants would have the background knowledge to span the gamut. Therefore, two separate meetings were decided upon, the first of which, Comparison of Mechanisms of Carcinogenesis by Radiation and Chemical Agents, was held at the National Bureau of Standards, Gaithersburg on December 6-7, 1983. The meeting was sponsored by the NCI. The second meeting has emerged as the present workshop at Brookhaven National Laboratory. An interface between the two conferences has been provided by Michael Fry and Donald Borg, who have summarized the salient points emanating from the NCI Symposium. We intended that the first conference should focus on the basic mechanisms of radiation and chemical carcinogenesis, while the second, the present meeting, would emphasize exposure-response relationships, particularly the theoretical and practical similar­ ities and differences between exposure to chemical carcinogens compared to exposure to ionizing radiation.