Mechanisms of anti-cancer effects of polyphenols, found in fruits, vegetables, spices and representing parts of daily nutrition, have been considered. These compounds may be the basis for development of cancer preventive preparations. They can block carcinogenesis initiation by inactivation of exogenous or endogenous genotoxic molecules including reactive oxygen species. Another mechanism consists in inhibition of activity and synthesis of carcinogen-metabolizing enzymes. Plant polyphenols also induce expression of antioxidant and detoxification enzymes genes.