Long-term oxygen therapy (LTOT) is a routine treatment method of chronic respiratory insufficiency. An indication for such treatment is PaO₂ (arterial partial oxygen tension) ≤55 mmHg or PaO₂ 56–60 mmHg when it is accompanied by one from three symptoms: radiographic signs of pulmonary hypertension, signs of right ventricle hypertrophy found in ECG and/or haematocrit ≥55%. To obtain positive results of oxygen therapy, patients should breathe oxygen at least 15 hours a day, including all night. Low flow rates (1-3 l/min) of supplemental oxygen are usually recommended. Three types of oxygen sources are now attainable: bottles with compressed oxygen, bottles with liquid oxygen, and, the most popular, oxygen concentrators. Patients are qualified for LTOT in special oxygen therapy centres. If we do not obtain satisfactory increase of PaO₂ in a chronic respiratory insufficiency patient, non-invasive mechanical ventilation should be considered. This method is mostly used in hospitals, to treat both acute and chronic respiratory insufficiency. Small mobile respirators are also available, so NIV can be used at the patient’s home.