Oxygen is essential for life, participates in biochemical reactions, enables the oxidation of nutrients and the generation of energy in form of ATP. Oxygen is also a very reactive molecule that can harm the body by producing free radicals. Free radicals are very unstable species that have one or more unpaired electrons in the outer shell, their halflives are very short and they tend to stabilize quickly. Therefore, they instantly react with other molecules in their immediate environment, thus starting a chain reaction in which new radicals are formed. Biochemically, the most significant representatives of free radicals are reactive oxygen species and reactive nitrogen species. Oxygen radicals are the most reactive free radicals with the greatest negative effects upon biological molecules. Tissue damage caused by oxygen radicals is often referred to as oxidative damage. The consequences of oxidative damage are many diseases. Substances that protect the body from harmful effects of oxygen radicals are known as antioxidants. Antioxidants are natural or synthetic substances that successfully neutralize free radicals. Man has evolved protective mechanisms of antioxidant defense. Sources of antioxidants can be endogenous (enzymatic) and exogenous (non-enzymatic). Key endogenous antioxidants are: superoxide dismutase, catalase, glutathione peroxidase and glutathione reductase. Nonenzymatic antioxidants include metabolic antioxidants and antioxidants from foods. Food antioxidants greatly help endogenous antioxidants in protecting the body from oxidative stress. Therefore, a varied and balanced diet as a fundamental source of antioxidants is the key to protecting the body from all harmful mechanisms.