Constituents from food, beverages and medicinal herbs have been studied extensively as cancer preventive agents. This article uses vitamin E (a nutrient) and tea polyphenols (non-nutritive phytochemicals) as examples to illustrate the opportunities, problems and lessons learned in this research area. The hypothesis that vitamin E prevents cancer is derived from epidemiological observations, but large scale human intervention trials with α-tocopherol (the most commonly recognized form of vitamin E) produced disappointing results due to our lack of understanding of the biological activities of different forms of tocopherols. The hypothesis that green tea prevents cancer has strong supportive evidence from laboratory studies, but only limited evidence from human studies. Even with these inconsistencies, many important advances have been made and lessons learned in the field of cancer prevention by phytochemicals. In future studies, more integration of laboratory and human studies are needed to advance the field of cancer chemoprevention.