Tea is particularly rich in polyphenols, including catechins, theaflavins and thearubigins, which are thought to contribute to the health benefits of tea. Tea polyphenols act as antioxidants in vitro by scavenging reactive oxygen and nitrogen species and chelating redox-active transition metal ions. They may also function indirectly as antioxidants through 1) inhibition of the redox-sensitive transcription factors, nuclear factor-kappaB and activator protein-1; 2) inhibition of "pro-oxidant" enzymes, such as inducible nitric oxide synthase, lipoxygenases, cyclooxygenases and xanthine oxidase; and 3) induction of phase II and antioxidant enzymes, such as glutathione S-transferases and superoxide dismutases. The fact that catechins are rapidly and extensively metabolized emphasizes the importance of demonstrating their antioxidant activity in vivo. Animal studies offer a unique opportunity to assess the contribution of the antioxidant properties of tea and tea polyphenols to the physiological effects of tea administration in different models of oxidative stress. Most promising are the consistent findings in animal models of skin, lung, colon, liver and pancreatic cancer that tea and tea polyphenol administration inhibit carcinogen-induced increases in the oxidized DNA base, 8-hydroxy-2'-deoxyguanosine. In animal models of atherosclerosis, green and black tea administration has resulted in modest improvements in the resistance of lipoproteins to ex vivo oxidation, although limited data suggest that green tea or green tea catechins inhibit atherogenesis. To determine whether tea polyphenols act as effective antioxidants in vivo, future studies in animals and humans should employ sensitive and specific biomarkers of oxidative damage to lipids, proteins and DNA.