A diagnosis that seems to cause much confusion in the clinical situation is that of a pneumothorax, which is the presence of air in the pleural space. Air is not normally present in this space. The visceral and parietal pleura are exclusively in contact with each other except for a very thin layer of intervening fluid which is not normally visible radiographically (Fig. 17.1). The expected appearance of a pneumothorax can be anticipated by using the basic principles which have already been introduced.