A spontaneous pneumothorax is one that occurs without any antecedent traumatic or iatrogenic cause. Spontaneous pneumothoraces can be categorized into primary or secondary types. A primary spontaneous pneumothorax occurs without any clinically apparent underlying lung disease known to promote a pneumothorax. The most common cause of primary spontaneous pneumothorax is an apical subpleural bleb. The typical patient is young, tall, and thin, and in late adolescence or early adulthood. The male-to-female ratio is 6:1. Clinical presentation may vary from asymptomatic (other than the initial chest pain episode) to tension pneumothorax with hemodynamic compromise or collapse. A secondary spontaneous pneumothorax occurs in lungs with underlying pathology that predisposes to a pneumothorax, such as cystic fibrosis, Marfan’s syndrome, metastatic osteosarcoma, Pneumocystis carinii, and asthma. Bullae are larger than blebs, usually larger than 1 cm. Rarely, a spontaneous pneumothorax may present with hemothorax due to disruption of vascular adhesions. The most common presentation, however, is without progression to tension pneumothorax. Symptoms also depend on the underlying baseline condition of the lungs, or of the remaining inflated lung. The plain chest x-ray is the standard diagnostic modality, preferably in the upright posteroanterior projection.