Atelectasis is still an important postoperative pulmonary complication which threatens the patient who has undergone major surgery. It is generally agreed that the predominant causal factor is a complete bronchial occlusion by secretions, although reflex nervous stimuli have also been implicated to contribute greatly to the occlusion. The role of postoperative hypoventilation in the etiology of atelectasis remains a controversial one. The diagnosis of atelectasis depends chiefly upon a study of the signs and symptoms, but is aided by the findings of physical examination, x-ray films, and occasionally, bronchoscopy. Treatment and prevention of atelectasis are simple and effective, consisting chiefly of measures to remove the obstruction, such as, coughing, postural drainage, or bronchial catheterization.