Through the use of molecular methods and mathematical models, epidemiologists are contributing to the improved understanding of the mechanisms of cancer. Multistage models with their mechanistic basis have been useful in descriptions of initiator-promoter type behavior of some carcinogens as well as genetic predisposition to rare tumors and reproductive risk factors in breast cancer. The use of biochemical and molecular laboratory techniques on tissue and fluid samples should provide important information in the near future concerning the basic mechanisms of human cancer. The potential of these methods is not only to describe exposure to carcinogens but also to indicate various host factors and their relevance to the risk of cancer.