Epidemiological studies have suggested that frequent consumption of cruciferous vegetables is associated with a decreased risk in various types of cancer. Cruciferous vegetables are commonly consumed foods that contain organosulfur compounds known as isothiocyanates. These compounds are potent inhibitors of chemically induced carcinogenesis in animals. Extensive work has been conducted to elucidate the mechanisms involved in the inhibition of carcinogenesis by isothiocyanates. These mechanisms include blocking the metabolic activation of the carcinogens by way of altering the enzymes involved in the process, induction of detoxification enzymes and induction of apoptosis. Since their mode of action is selective, the enzyme composition of the tissue and the inhibition or induction of the enzymes by the isothiocyanates will influence their chemopreventive activities. Isothiocyanates may potentially be beneficial in protecting against human carcinogenesis.