Humans are susceptible to the carcinogenic action of a small group of organic and inorganic chemicals in certain industrial, medical, and social habit exposures. A larger number and wider variety of chemical carcinogens, primarily organic compounds, are known for experimental animals. Chemical carcinogens are also found among the metabolites of living cells. No common structure is evident among chemical carcinogens, and a majority of these agents are precarcinogens that require metabolic activation into reactive electrophilic ultimate carcinogens. These strong electrophiles combine covalently with nucleophilic sites in DNAs, RNAs, and proteins in target tissues. One or more of these adducts appear to initiate carcinogenesis. About 20 naturally occurring organic chemical carcinogens, primarily metabolites of green plants and fungi, are known; some occur in some human foods. Many other naturally chemical carcingens doubtless exist among the vast number of uncharacterized nonnutritive minor components of living systems, some of which are sources of human foods. The electrophilic forms of chemical carcinogens are mutagenic, and mammalian tissue-mediated mutagenicity assays appear promising in the detection of potential chemical carcinogens. These assays should serve at least as a prescreen for conventional lifetime tests in rodents for the carcinogenic activity of food components and contaminants. Epidemiological approaches appear necessary to evaluate the importance of the naturally occurring chemical carcinogens in the occurrence of human cancer.