Antioxidants are recognised as important components in human and animal nutrition. They also serve as functional ingredients in the food industry, for example, to reduce the oxidative deterioration of oils, fats and lipid based foods. Antioxidants are believed to protect cells and organs from oxidative stress and therefore act beneficially against a number of diseases. Although often seen to exhibit antioxidant activity in vitro, it is unlikely that some bioactive components, such as polyphenols and other phytochemicals, retain those beneficial antioxidant properties when ingested. Indeed, they may act as pro-oxidants with resulting negative consequences. However, they do play an important role and understanding these food components is essential in recognising how they can best be exploited by the industry as processing aids and for improving nutritional value. Making optimal choices to maximise effects for their intended use is paramount in producing cost effective products. Additionally, legislation surrounding this field raises some important challenges with regards to health claims substantiation and the use and marketing of antioxidant functional ingredients.