Both essential and nonessential dietary antioxidants have been suggested to be beneficial for health. However, studies show that many antioxidants can also exhibit prooxidant behavior under certain conditions. The antioxidant/prooxidant activity of phenolics is dependent on such factors as metal-reducing potential, chelating behavior, pH, and solubility characteristics. These factors, in addition to bioavailability and stability in tissues, should be considered when evaluating the potential bioactivity of dietary phenolics.