Cancer prevention through the use of chemical intervention regimens (chemoprevention) is an emerging field with broad potential for impacting on cancer incidence rates in defined high-risk groups and the general population. Information from cancer epidemiologic studies coupled with that from basic research on cancer biology have combined to reveal several categories of agents with potential for clinical application, including natural and synthetic tumor suppressive retinoids and antioxidants. Chemopreventive agents may inhibit the development of cancer by limiting exposure to initiators or promoters through stimulation of inactivation or excretion mechanisms. Biological consequences of exposure to carcinogens may also be interfered with, e.g., by inhibiting the activation of proto-oncogenes or by antagonizing the effects of oncogene expression. Hundreds of compounds with chemopreventive efficacy in vitro have been isolated from foods and plant products. The testing and development of candidate chemopreventives proceeds through a series of preclinical efficacy screens, followed by controlled clinical trials.