Polyphenols having catechol (1,2-dihydroxybenzene) nuclei are strong in vitro antioxidants owing to their ability to rapidly reduce reactive oxygen species (ROS), bind transition metal ions as inert complexes and regenerate the potent chain-breaking antioxidant alpha-tocopherol. Polyphenols provide antioxidant protection to plants by acting as electron donors to peroxidases that reduce hydrogen peroxide into water. Polyphenols are also likely to improve the oxidative stability of food emulsions by binding metal ions and/or reducing ROS at phospholipid or protein interfaces. This protection may extend to the gastric compartment where millimolar concentrations of polyphenols can accumulate after consumption of a meal rich in plant products. Beyond the digestive tract, direct antioxidant effects become quite hypothetical as polyphenols are only poorly absorbed and mostly converted into weakly reducing metabolites. However, despite the low circulating concentrations of polyphenol metabolites, ROS-scavenging and/or inhibition of ROS-producing enzymes could still operate provided that polyphenol metabolites accumulate on the very sites of oxidative stress and inflammation. Finally, oxidation of polyphenols in cells can lead to electrophilic/oxidizing metabolites with a capacity to up-regulate genes coding for antioxidant enzymes. In conclusion, the ability of polyphenols to act as antioxidants in humans must be severely qualified.