Food contains a large number of inhibitors of carcinogenesis, including phenols, indoles, aromatic isothiocyanates, methylated flavones, coumarins, plant sterols, selenium salts, protease inhibitors, ascorbic acid, tocopherols, retinol, and carotenes. The diversity and widespread occurrence of these compounds in food make it virtually impossible to consume a diet that does not contain inhibitors of carcinogenesis. Inhibitors can be classified as to the time in the carcinogenic process at which they act. Some prevent formation of carcinogens. Others, termed "blocking agents," prevent carcinogens from reaching or reacting with critical target sites. A third group called "suppressing agents" are effective when fed subsequent to administration of carcinogens. Some compounds inhibit at more than one time point. The major emphasis in this paper is on blocking agents, in particular those that act by enhancing host detoxification systems. Mary blocking agents produce a coordinated enhancement of multiple detoxification systems. Two distinctive patterns termed type A and type B have been identified. One enzyme system commonly induced by blocking agents is glutathione S-transferase. On the basis of this information, induction of glutathione S-transferase activity is being used to detect the presence of blocking agents in complex natural products. Green coffee beans induce increased glutathione S-transferase activity and inhibit mammary neoplasia in the rat resulting from administration of 7,12-dimethylbenz(a)anthracene. Two potent inducers of increased glutathione S-transferase activity have been isolated from green coffee beans. These are kahweol palmitate and cafestol palmitate. In recent work, several plant materials have been found to inhibit carcinogenesis when fed after carcinogen exposure. The identification and further investigation of inhibitors present in food are of importance so that their impact on the occurrence of neoplasia in humans can be ascertained.