Flavonoids represent a major group of natural antioxidants. In recent years, flavonoids have attracted increasing interest due to their various beneficial pharmacological effects and additional abilities to modulate CYPs and P-glycoprotein (Pgp), the product of mdr (multidrug - resistance) genes. There is a very large amount of in vitro data of flavonoids, but very few reports of animal studies are available. Moreover, information concerning the biological fates of flavonoids is very limited. Therefore, whether published in vitro data are predictive of in vivo effects need further discussion. On the other hand, flavonoids may activate or inhibit CYPs and Pgp, which may be beneficial in detoxication, in chemoprevention, or in drug resistance suppression. The aim of this review is to give an overview of the research reports on the biological fates of flavonoids and their potential interactions with xenobiotics. The perspectives for future research on flavonoids in herbs will be suggested at the end of this article.