Quercetin is considered as one of the most abundant flavonoids and represents an integral part of human diet. High amounts of quercetin are found in numerous vegetables, fruits, nuts, tea, seeds and wine. It exhibits a wide range of biological activities, including anti-inflammatory anti-carcinogenic and antiviral actions. The chemoprevention elicited by polyphenols such as Q is mediated by apoptosis in tumor cells through direct activation of caspase cascade (mitochondrial pathway) such as caspase 3 and 9 as well as Bcl-2 family members but is without effect on their normal cell counterparts. Moreover, it exerts a notable cytoptotection activity against t-BHP-induced oxidative stress in HepG2 cells. Epigallocatechin-3-gallate is found in high amounts in green tea (Camellia sinesis), a popular drink throughout the world. EGCG is a potent chemopreventive agent, protecting against many types of cancer such as those induced by chemicals or radiation hepatic injury as well as protecting cells from damage induced by free radicals. The protective role of EGCG is selective, its effect being achieved in cancer cells but not in normal cells. These differences in EGCG actions may be attributed to the differences between antioxidant defence mechanism in normal cells and mechanism of oxidative stress in cancer cells.