Tea polyphenols (TPP) are key contributors to the beneficial health effects of green tea and black tea. However, their molecular mechanisms of action remain unclear. This article discusses the importance of the bioavailability of TPP in understanding their mechanisms of action and health effects of tea consumption. The systemic bioavailability is rather high for smaller catechins, low for galloyl catechins, and very low or null for oligomers and polymers from black tea. The bioavailability of TPP oxidation-derived polymers and self-assembled nanomaterials is not clearly known. If the large molecular weight TPP cannot get into systemic circulation, then the biological activities and mechanisms of action derived from studies in vitro are unlikely to be relevant to their actions in internal organs in vivo. In that case, their interactions with microbiota and actions on the epithelial cells of the gastrointestinal tract are important to their health effects. Therefore, the bioavailability of different types of TPP is an important factor in determining their mechanisms of action and the health effects of tea consumption.