Considerable information on the carcinogenic potential of chemical and radiological agents has accumulated from the epidemiological and toxicological studies conducted to date. In this article, we discuss dose-response relationships in carcinogenesis from both empirical and theoretical points of view. Emphasis is placed on the application of biologically based models to describe observed dose-response relationships for exposure to single and multiple agents known to increase cancer risk. The implications of these observations for inferences about possible mechanisms of carcinogenesis are explored.