Flavonoids and phenolic acids are widely distributed in higher plants and form part of the human diet. Recent interest in these substances has been stimulated by the potential health benefits arising from the antioxidant activity of these polyphenolic compounds. This review outlines the basic chemistry, biosynthesis, and structure-activity relationships of these compounds with respect to their antioxidant activity. Although there is considerable in vitro evidence establishing antioxidant activity for polyphenolics found in the diet, there are few studies in humans on the absorption and bioavailability of these compounds. The possible in vivo antioxidant effects of the flavonoids is even less well understood. For example, controlled human intervention studies with beverages, such as red wine, that are rich in polyphenolic compounds, have yielded conflicting results. Our own work and that of others suggests that the final effects of such beverages may be a balance between the well-described prooxidant effects of alcohol and its metabolism and the antioxidant effects of the polyphenolic constituents. There is a need for further studies to increase our understanding of the absorption and in vivo biological effects of this family of compounds.