Resveratrol (3,4',5-trihydroxystilbene, RESV) is a natural phenolic compound that exists as cis and trans isomers [c-RESV or (Z)-RESV and t-RESV or (E)-RESV, respectively]. t-RESV is a natural component of Vitis vinifera L. (Vitaceae), abundant in the skin of grapes (but not in the flesh) and in the leaf epidermis, and present in wines, especially red wines. In in vitro, ex vivo and in vivo experiments t-RESV exhibits a number of biological activities, including anti-inflammatory and anticarcinogenic properties. RESV also exists in wines as a cis isomer, which (unlike t-RESV) is not currently available commercially; as a result, little is known about this isomer's pharmacological activity. In this review, I will focus on the few comparative studies of the antioxidant effects of the two RESV isomers in different experimental models.