Hemothorax is the accumulation of blood in the pleural space, most commonly from trauma (rib fractures lacerating intercostal or internal mammary arteries, lung parenchymal injury, or great vessel injury). Non-traumatic causes include malignancy, pulmonary embolism with infarction, coagulopathy/anticoagulation, ruptured aortic aneurysm, and spontaneous hemopneumothorax. The pleural space can hold 2-3 liters of blood — massive hemothorax (> 1500 mL or > 200 mL/hr for 2-4 hours) constitutes a surgical emergency. A retained hemothorax that is not evacuated within 72 hours becomes organized as fibrin deposits trap blood, forming a fibrous peel (fibrothorax) that restricts lung expansion and may become infected (empyema). The decision between chest tube drainage alone versus surgical intervention (VATS or thoracotomy) depends on the initial output and ongoing drainage rate.