High intake of fruit and vegetables is believed to be beneficial to human health. Fruit, vegetables and some beverages, such as tea and coffee, are particularly rich in dietary polyphenols. Various studies have suggested (but not proven) that dietary polyphenols may protect against cardiovasucalar diseases, neurodegenerative diseases and some forms of cancer. Dietary polyphenols may exert their anticancer effects through several possible mechanisms, such as removal of carcinogenic agents, modulation of cancer cell signaling and antioxidant enzymatic activities, and induction of apoptosis as well as cell cycle arrest. Some of these effects may be related, at least partly, to their antioxidant activities. In recent years, a new concept of the antioxidant effects of dietary polyphenols has emerged, i.e., direct scavenging activity toward reactive species and indirect antioxidant activity; the latter activity is thought to arise primarily via the activation of nuclear factor-erythroid-2-related factor 2 which stimulates the activities of antioxidant enzymes such as glutathione peroxidase (GPx), glutathione S-transferase, catalase, NAD(P)H: quinone oxidoreductase-1 (NQO1), and/or phase II enzymes. The direct antioxidant activity of dietary polyphenols in vivo is probably limited because of their low concentrations in vivo, except in the gastrointestinal tract where they are present in high concentrations. Paradoxically, the pro-oxidant effect of dietary polyphenols may contribute to the activation of antioxidant enzymes and protective proteins in cultured cells and animal models because of the adaptation of cells and tissues to mild/moderate oxidative stress. Despite a plethora of in vitro studies on dietary polyphenols, many questions remain to be answered, such as: (1) How relevant are the direct and indirect antioxidant activities of dietary polyphenols in vivo? (2) How important are these activities in the anticancer effects of dietary polyphenols? (3) Do the pro-oxidant effects of dietary polyphenols observed in vitro have any relevance in vivo, especially in the potential anticancer effect of dietary polyphenols? Apparently, more carefully-designed in vivo studies are needed to answer these questions.