Reactive oxygen species and reactive nitrogen species are formed in the human body. Endogenous antioxidant defences are inadequate to scavenge them completely, so that ongoing oxidative damage to DNA, lipids, proteins and other molecules can be demonstrated and may contribute to the development of cancer, cardiovascular disease and possibly neurodegenerative disease. Hence diet-derived antioxidants may be particularly important in protecting against these diseases. Some antioxidants (e.g. ascorbate, certain flavonoids) can exert pro-oxidant actions in vitro, often by interaction with transition metal ions. The physiological relevance of these effects is uncertain, as is the optimal intake of most diet-derived antioxidants. In principle, these questions could be addressed by examining the effects of dietary composition and/or antioxidant supplementation upon parameters of oxidative damage in vivo. The methods available for measuring steady-state damage (i.e. the balance between damage and repair or replacement of damaged molecules) and the actual rate of damage to DNA, proteins and lipids are reviewed, highlighting areas in which further methodological development is urgently required.