Oxygen derived free radicals (OFR) arise in the course of normal cellular life, especially during cellular respiration. They are formed when molecular oxygen is reduced to water. These highly reactive species are controlled by a protective system both enzymatic and non enzymatic which helps to prevent the accumulation of peroxidative damage to the cell. Lipid peroxides result from the reaction of oxygen derived free radicals with polyinsatured fatty acids of membranes phospholipids and can be formed both non enzymatically and enzymatically (eicosanoids). Oxygen derived free radicals attack over-running beyond the protective system leads to oxidative stress. The cells involved in inflammation (polymorphonuclear, leucocytes, monocytes, platelets, endothelial cells) release oxygen derived free radicals and lipid peroxides and inflammatory diseases of infectious or non infectious origin can be considered as oxidative stress. Intracellular oxidative stress can lead to cellular death or trigger a strong inflammatory reaction. This occurs during ischemia reperfusion injury and hyperoxia. Exposure to ionizing radiation results in overproduction of oxygen derived free radicals both extra and intracellular. Oxidative stress may be involved in atheroma (where oxidised LDL are described), ageing and cancer.