Treatment of respiratory insufficiency with retention of CO2 by mechanic ventilation has come into use over the last decade, favored by use of non-invasive methods like nasal ventilation. Best results have been observed in hypercapnic respiratory insufficiency caused by neuromuscular disease or restrictive pathologic changes of the lung. Nocturnal use of nasal ventilation alone is often sufficient to correct also the daily CO2-values. Mechanisms explaining this beneficial effect are not yet known-The respiratory CNS-centers, respiratory muscles or thoracopulmonary mechanics may play etiologically an important role. Medical indications for nasal ventilation in chronic obstructive pulmonary disease are not clear, since results from several studies are controversial. Also in severe or progressive neurologic diseases a critical evaluation is mandatory for assessment of benefits including improvement of quality of life.