Diets rich in polyphenols are epidemiologically associated with lower risk of developing some age-related diseases in humans. This apparent disease-protective effect of polyphenols is often attributed to their powerful antioxidant activities, as established in vitro. However, polyphenols can also exert pro-oxidant activities under certain experimental conditions. Neither pro-oxidant nor anti-oxidant activities have yet been clearly established to occur in vivo in humans, nor are they likely given the limited levels of polyphenols that are achievable in vivo after consumption of foods and beverages rich in them. Other actions of polyphenols may be more important in vivo. Many studies of the biological effects of polyphenols in cell culture have been affected by their ability to oxidise in culture media, and awareness of this problem can avoid erroneous claims.