A relatively new application of emergency ultrasound is its use in the diagnosis of pneumothorax. In patients with major trauma, early detection and treatment of pneumothorax are vital. Chest radiography in these patients is limited to anteroposterior (AP) supine films, in which radiographic features of pneumothorax may be quite subtle. Hence, rapid and accurate bedside ultrasonography can expedite resuscitation. Sonographic features of pneumothorax have been identified in a number of studies. The technique involves identification of the pleural line and observation for features such as "lung sliding" and comet-tail artifacts, which are absent in pneumothorax. Based on a review of the literature, the author describes these features and discusses the utility of emergency ultrasound in detecting pneumothorax.