Beneficial health effects of tea have been demonstrated in animal experiments and some human studies. The two most extensively investigated diseases are cancer and heart disease. Although mechanisms of protective activity of tea against these diseases have been proposed, there are inconsistencies in the relationship between tea consumption and the risk of these diseases in humans. The bioavailability of active components is beginning to be understood, but further research is required to determine whether the results from animal studies are applicable to humans. Also discussed are the possible effects of tea in increasing thermogenesis and bone density as well as decreasing risk of cataracts and arthritis. The potential health benefits of tea consumption warrant further investigation.