This review discusses the different aspects of long-term oxygen therapy (LTOT). In selected patients with chronic hypoxaemia, LTOT may increase the life-span and quality of life. Since identification of these patients is both important and difficult, the indications for LTOT are discussed. Next, the appropriate time at which LTOT should be initiated is discussed in the section on the physiological criteria. Even in patients who qualify for LTOT, the effectiveness of this therapy may vary. Because they are important in this context, the prescription guidelines are considered. Finally, possible ways of increasing the efficiency of LTOT are reviewed in the sections on home oxygen systems, oxygen-conserving methods and the organization of LTOT in the Netherlands.