Antioxidants can be defined on a biological basis by their ability to inhibit the oxidation of susceptible biomolecules such as proteins, lipids and DNA. There is observational evidence on an inverse association between dietary antioxidant intake and the development of chronic conditions, although randomized trials have not confirmed these results. The antioxidant potential of a biological material can be evaluated by methods categorized into three groups. The first group includes the determination of the individual antioxidants present in the investigated sample. The second focuses on 'total antioxidant capacity', which is considered a convenient tool for obtaining an overall estimate of the antioxidant potential of the sample. The third group involves the quantification of in vivo markers of oxidative stress, such as the products of protein, lipid and DNA oxidation. Due to differences between the assays, however, current research suggests the application of a combination of such methods for a more informative assessment of the oxidation status of the organism.