Among diet antioxidants, polyphenols, naturally occurring in vegetables, fruits and plant-derived beverages such as tea, red wine, and extra virgin olive oil, are the most abundant ones. In vitro cell culture experiments have shown that polyphenols possess antioxidant properties, and it is thought that these activities can contribute to the prevention of several oxidative stress-associated diseases. It has however become clear that the mechanisms of action of polyphenols go beyond their intrinsic reducing capabilities, being able to exert other additional effects that are as yet poorly understood. This article gives an overview of the most recent data on the subject and describe the additional functions that polyphenols can have in biological systems, focusing on their effects on glutathione and its related enzymes. Evidence is provided of a tight connection between exogenous and endogenous antioxidants that appear to act in a coordinated fashion. Experimental data indicate that polyphenols may offer an indirect protection by activating endogenous defense systems. It is reasonable to hypothesize that this is achieved, at least in part, through antioxidant responsive elements (ARE) present in the promoter regions of many of the genes inducible by oxidative and chemical stress. The latest studies strongly suggest that dietary polyphenols can stimulate antioxidant enzyme transcription through ARE.