Oxygen therapy is justified both on theoretical grounds and by clinical studies. Chronic hypoxia bodes ill for the system and is a prognostic factor in pulmonary disease. Low flow oxygen therapy has not shown any risk of pulmonary toxicity from anatomical or physiological studies. Clinical studies have shown that the correction of hypoxaemia by long term oxygen therapy improves exercise tolerance, mental state, the general sense of well being, polycythaemia, pulmonary hypertension, the quality of sleep, and finally the prognosis. But long term oxygen therapy is costly and requires patient co-operation and close supervision. It should be reserved for hypoxic patients in a stable state: the exact degree of hypoxaemia at which oxygen therapy is permissible cannot be defined precisely and depends on other criteria (such as polycythaemia, pulmonary arterial hypertension, nocturnal desaturation). Account should be taken of the PaCO2 level and the cause of the disease in deciding the oxygen flow. Polycythaemia, pulmonary arterial hypertension, nocturnal desaturation despite a normal waking PaO2, may represent some indications for oxygen therapy but further studies are necessary.