Epidemiological studies suggest that brassica vegetables are protective against cancers of the lungs and alimentary tract. Cruciferous vegetables are the dietary source of glucosinolates, a large group of sulfur-containing glucosides. These compounds remain intact unless brought into contact with the enzyme myrosinase by pests, food processing, or chewing. Myrosinase releases glucose and breakdown products, including isothiocyanates. These highly reactive compounds are potent inducers of Phase II enzymes in vitro. Isothiocyanates also inhibit mitosis and stimulate apoptosis in human tumor cells, in vitro and in vivo. To understand and exploit such effects it is important to determine the routes of absorption of glucosinolate breakdown products, their metabolism, and delivery to systemic tissues. Glucosinolates can be gained or lost by vegetables during storage. They may be degraded or leached during processing, or preserved by thermal inactivation of myrosinase. Glucosinolates are broken down by plant myrosinase in the small intestine or by bacterial myrosinase in the colon. Isothiocyanates are absorbed from the small bowel and colon, and metabolites are detectable in human urine two to three hours after consumption of brassica vegetables. Interpretation of epidemiological data and exploitation of brassica vegetables for human health requires an understanding of glucosinolate chemistry and metabolism, across the whole food chain, from production and processing to the consumer.