Microbial transformation of triterpenoids has provided new derivatives that are potentially useful for pharmacological studies. In these biotransformation processes, several reactions that are difficult to achieve by chemical means have been accomplished, such as: introduction of hydroxyl groups into remote positions of the molecules; selective cleavage of the side chains of tetra-cyclic terpenoids to produce C19 steroids; regioselective glycosidic transfer reactions; selective ring cleavage through a Baeyer-Villiger-type oxidation to ren-der seco-triterpenoids; and carbon skeleton rearrangements involving a methyl group migration. These biotransformations have also been used as in vitro models to mimic and predict the mammalian metabolism of biologically active triterpenoids.