Green tea (Camellia sinensis, Theaceae) and its major polyphenol constituents, the catechins, have been reported to have many health benefits including the prevention of cancer and heart disease. Many mechanisms of action have been proposed based on in vitro models; however, the importance of most of these mechanisms remains to be determined in vivo. The bioavailability and biotransformation of tea catechins play a key role in determining the importance of various mechanisms in vivo. Likewise, the biological activity and bioavailability of tea catechin metabolites, an understudied area, are important in understanding the potential beneficial effects of tea. In this article, we review the data available on the biotransformation of the tea catechins and the limited data set available on the biological activities of the catechin metabolites. Careful interpretation of available data, carefully designed animal experiments, and integration of bioavailability and biological activity data are needed if the disease preventive activity of tea is to be understood. We hope this article will spark research efforts on some of the important questions regarding tea polyphenol bioavailability, biotransformation, and the biological activities of tea catechin metabolites.