When a main bronchus or one of its branches is occluded, then part of the lung is put out of action. Interest in the resulting clinical picture has increased during recent years. The causes leading to such an occlusion are particularly frequent in children. Now that attention is being drawn more and more to this affection, it appears that lung collapse is fairly often found in hospital practice. Often it is possible to make a diagnosis at an early stage; radiological examination is of great importance in this connection. In this way a better insight has béen obtained into the different clinical forms of lung collapse and into the causes leading up to it. This is not only of importance from a diagnostic point of view, but also therapeutically, as undoubtedly in many cases bronchiectasis will develop in the collapsed lung unless the occlusion is remedied in good time.