The thoracic or pleural cavity is the potential space between the lungs, mediastinum, diaphragm and thoracic wall. It is lined by the pleura, a serous membrane that is described by the particular structure that it covers. The visceral pleura covers the lungs, whilst the parietal pleura lines the rest of the thoracic cavity and is further classified as costal, diaphragmatic or mediastinal. The mediastinum is the central tissue partition of the thoracic cavity that separates the two hemithoraces. Controversy exists as to whether the mediastinum is a complete or a fenestrated structure; regardless of this, it is easily disrupted and a unilateral disease process typically affects the contralateral side. The mediastinum has anatomical boundaries of the thoracic inlet, diaphragm, thoracic spine and sternum; it encloses the heart, aorta, trachea, mainstem bronchi, oesophagus, thymus, thoracic duct, and phrenic and vagus nerves. Anatomically, the mediastinum is divided into cranial, middle and caudal portions. The chapter looks at Clinical signs; Diagnostic approach; Thoracostomy tube placement; Pneumothorax and pneumomediastinum; Pyothorax; Haemothorax; Chylothorax; Neoplastic effusion; and Mediastinal masses.