A basic understanding of both the gross and roentgenologic anatomy of the normal chest is essential to early recognition and accurate evaluation of any disease process within the chest. It is the purpose of this paper, however, merely to emphasize the roentgen appearance of certain structures which are likely to change in shape, size, position, or contour when a lung, or any part of a lung, becomes collapsed. While some changes are only suggestive of, or consistent with, a decrease in size of a lung or any part thereof, others, when clearly demonstrated, are almost pathognomonic of the presence of collapse. Our observations are based on a detailed study of approximately 1,200 cases of tumor, bronchiectasis, foreign body, and tuberculosis. The only cases of tuberculosis that are included, however, are those in which a decrease in size of an involved lobe was marked. Postero-anterior and lateral roentgenograms on 160 healthy young adult hospital employees were also examined in an attempt to establish the variations which could be considered normal. In 10 of this group, roentgenograms were taken during both full inspiratory and full expiratory phases of respiration. In approximately 300 of the abnormal group complete bronchographic studies were available; in 3 other persons a complete bronchographic examination during both full inspiration and full expiration was made. Because of the similarity of the anatomic structures of the two sides of the chest, each roentgenogram offers an immediate and, in the majority of cases, reliable means of discovering unilateral abnormalities. Comparison of an abnormal lung with its normal opposite will often make possible accurate diagnosis of collapse of a lobe or a segment of a lobe. Heretofore, the variations from normal usually accepted as diagnostic of collapse of the lung were: (1) an abnormal shadow of increased density, (2) elevation of the diaphragm, (3) displacement or shift of the mediastinum, and (4) narrowing of the rib spaces. These variations have been thoroughly studied and presented in the past and require no further discussion at this time. Our study has demonstrated the importance of three additional anatomic factors: (1) the appearance and position of the hilar shadows, (2) the arrangement of the vascular shadows in the peripheral portions of the lung fields, and (3) the demonstration of the actual size of a lobe as determined by the appearance and position of the septa or fissures of the lung. Anatomically, the hili are approximately at the same level, but roentgenologically the left hilus usually appears to be a few millimeters higher than the right (Fig. 1). This is due, for the most part, to the fact that the left pulmonary artery, which forms the upper margin of the left hilar shadow, is more clearly visualized than the eparterial right main bronchus, which forms the upper margin of the right hilus.