The aetiology, pathogenesis and radiographic signs of pneumomediastinum are reviewed. A series of seventeen cases in dogs is reported. In these seventeen dogs thoracic trauma was the most common cause of pneumomediastinum. The most frequent and reliable radiographic signs were, delineation of both the inner and outer aspects of the tracheal wall, identification of the azygos vein and the presence of mediastinal air pockets. Pneumothorax was also present in fifteen cases. The radiographic appearance of pneumomediastinum is compared with that of oesophageal air.