Pneumomediastinum, also known as mediastinal emphysema, represents extraluminal gas in the mediastinum. Pneumomediastinum can lead to pneumothorax, pneumopericardium, pneumoperitoneum, or pneumoretroperitoneum. There are many causes of pneumomediastinum (Table 1) and several radiographic signs: pneumopericardium, continuous diaphragm sign, continuous left hemidiaphragm sign, Naclerio's V sign, V sign at confluence of brachiocephalic veins, ring-around-the-artery sign, thymic spinnaker-sail sign, and extrapleural air sign. We review the common and uncommon signs of pneumomediastinum and present some new signs. We also address the problem of distinguishing pneumomediastinum from pneumothorax and pneumopericardium.