Natural antioxidants like vitamin C, vitamin E, carotenoids, and polyphenols like flavonoids, are at present generally considered to be beneficial components from fruit and vegetables. The anti-oxidative properties of these compounds are often claimed to be responsible for various beneficial health effects of these food ingredients. Together these studies provide the basis for the present rapidly increasing interest for the use of natural antioxidants as functional food ingredients and/or as food supplements. However, at higher doses or under certain conditions antioxidant-type functional food ingredients may exert toxic pro-oxidant activities. The present manuscript gives an overview of especially this pro-oxidative chemistry and toxicity of well-known natural antioxidants including vitamin C, vitamin E, carotenoids and flavonoids.