Controlled studies have demonstrated that the correction of tissue hypoxia increases survival and reduces pulmonary hypertension in patients with chronic obstructive pulmonary disease (COPD) receiving oxygen therapy 15 h/day or longer. Long-term oxygen therapy (LTOT) is recommended to any patient with COPD who has a PaO2 of < or = 7.3 kPa. In most countries, the PaO2 threshold is 8kPa in patients with chronic hypoxemia (PaO2 > or = 55 mm Hg) with associated hematocrit > or = 55%, pulmonary hypertension or cor pulmonale. Desaturations during sleep or exercise should be investigated, although a consensus as to whether and how these episodes should be treated has yet to be reached. The indications for LTOT in restrictive lung diseases, such as interstitial pulmonary fibrosis and pneumoconiosis, remain controversial. In many countries, oxygen is not prescribed if the patient is a current smoker. Breathlessness without hypoxemia should not be considered an indication for LTOT. The oxygen is usually administered through nasal cannula. Venturi type masks, nasopharyngeal and transtracheal catheters are associated with several drawbacks. Oxygen is usually supplied by the relatively cheap oxygen concentrator. Liquid oxygen is favored when a portable source is an important requirement. Many questions remain unanswered concerning the duration of added survival, the effect of LTOT on physiological parameters such as pulmonary artery pressure, respiratory failure in non-COPD patients, exercise and nocturnal desaturations.