In this chapter, we review the existing data concerning the effect of polyphenols on the transport of some bioactive compounds (glucose, thiamine, folate and organic cations) at the intestinal and placental epithelial barriers. Important conclusions can be drawn. Firstly, different classes of polyphenols affect transport of these compounds at the intestine and placenta. Secondly, different compounds belonging to the same phenolic class often possess opposite effects upon transport of a given molecule. Thirdly, short- and long-term exposure to a given polyphenol does not produce parallel results. Fourthly, combination of distinct polyphenols, as happens in natural foods, may cause very distinct effects from the expected ones based on the effect of each of these compounds alone. Finally, ethanol is able to modify the effect of polyphenols. In summary, the studies reviewed here raise a concern about possible changes in the bioavailability of these substrates upon concomitant ingestion of polyphenols.