Free radicals are generated in a wide variety of chemical and biological systems, including the production of plastics, the ageing of paints, the deterioration of foods, the combustion of fuels and in the human body. In living organisms, the levels of free radicals and other ‘reactive species’ (such as hydrogen peroxide) are controlled by a complex web of antioxidant defences, which minimise (but do not completely prevent) oxidative damage to biomolecules. One reason for this is that reactive species play useful roles, for example, in cell signalling and especially in defence against pathogens. However, over the long human lifespan, oxidative damage may contribute to diseases (e.g. cancer, cardiovascular disease, dementia) and perhaps even to the ageing process itself. Some antioxidants come from the diet, whereas others (such as glutathione and the superoxide dismutase enzymes) are made in vivo.