The Use of Oxygen has proved to be highly beneficial in treatment of arterial hypoxemia, as it's considered not only as a medical gas but as a drug used in treatment.Nowadays, we've sound clinical studies evaluating benefits of short term (acute) and long term (chronic) oxygen therapy.Oxygen therapy has participated in treatment of almost all cases of respiratory critically ill patients, but before fully appreciating its use in these patients, we've to review:•The Physiology of oxygen and tissue oxygenation•Acute and chronic hypoxemia•Oxygen delivery systems and devices•Hazards of oxygen therapyOxygen is used to treat or prevent hypoxia, to achieve this goal, supplemental oxygen must be given in sufficient dosage to appropriate patients. Oxygen therapy should supply the needed metabolism and cellular functions can be maintained. In conditions of hypoxia due to inadequate oxygenation to normal or abnormal lungs, oxygen therapy is basic and relatively easy to achieve at no significant risk to the patient. But in other situations as cardiac failure, cyanide poisoning and anemic hypoxia, oxygen therapy can become complex and presents risk to the patient. So, we've to expand our goal to include the prevention of physiologic compensatory mechanisms secondary to hypoxia and provide adequate amount of oxygen to tissues without causing oxygen toxicity and oxygen induced hypoventilation.In the next pages some respiratory diseases encountered in the ICU will be discussed with the role of oxygen in its treatment.Also, hazards of oxygen will be discussed especially if hyperbaric or received for long time.