Oxygen has invaded progressively, and through the ages, an initially anaerobic world. Living organisms had to invent, in the course of evolution, diverse and ingenious defence systems, to survive the toxicity of this element, which was new for them. Strengthened by this experience over billions of years, the present superior organisms, and particularly human species, are thoroughly adapted to 21 per cent of atmospheric oxygen. Nevertheless, the equilibrium is fragile and the menace of oxygen hovers continually. This deleterious potential of oxygen is attributed to the formation, in vivo, of free radicals, a free radical being, by definition, any chemical species possessing one or several mismatched electrons. These free radicals are, in general, very active. They trigger chain reactions able to damage the different constituents of the living organism. Basic oxygen, must be pre-activated to manifest its toxicity. Such an activation can be achieved in two ways: it can be photodynamic, ending mainly in singlet oxygen, it can be reducing, followed by the formation of the anion hydrogen peroxide and of radical hydroxyl; the latter is the most reactive chemical species in the biological world. The reductive process is accelerated in the presence of transition metals, such as iron and copper, and/or specific enzymes (monoxygenase and certain oxydases). This activation takes place in different cellular compartments: mitochondria, microsomes, peroxysomes, cytoplasmic membrane. To this potential toxicity of oxygen the organism opposes different anti-oxidant defence systems. A first group works up the radical chain, inhibiting activation mechanisms. Such a group, as a consequence, warns of the initiation of radical reactions. The second group neutralizes the free radicals already formed and thus stops the chain of propagation. In this group can be found detoxifying enzymes, notably superoxide dismutase and catalase, producing jointly peroxidases, particularly peroxidase glutathions. Such enzymes for the most part have trace elements as cofactors. In this second group can also be found various molecules which act like 'substrate suicide', or as an anti-oxidant shield. Among these molecules, some can act in the lipidic phase, such as tocopherols, carotenoïds and ubiquinones. Other molecules which are lipophobic, mainly ascorbic acid and uric acid, are active in a hydrated environment. In the case of a weakening of such an antioxidant defence or excess production of radicals, a state of oxidative stress occurs. Uncontrolled, these radicals will damage different biological targets: lipids, DNA, proteins. Disturbances of cellular metabolism will occur, unless corrective defences intervene. The identification of these radical phenomena is an obligatory stage. But because of the very short life span of free radicals, identification poses a real analytical problem. However, three approaches are possible: identification of free radicals, either directly by means of paramagnetic electron resonance, or indirectly by identifying some more stable intermediates. evaluation of the traces of radical attack on biological molecules, for example by high performance liquid chromatography, gas-liquid chromatography, colorimetric tests, estimation of the antioxidant status, for example by colorimetric tests, immunoenzymatic methods, high performance liquid chromatography.