Pneumothorax occurs when air is present in the pleural space between the visceral and parietal pleura and is often graded in severity based on size, symptoms, and physiologic effects (i.e., hypoxemia, tachycardia, and hypotension). It is classified based on the trigger (spontaneous, traumatic, and iatrogenic) and on the underlying characteristics of the patient, particularly the presence of underlying lung disease (secondary) or not (primary). Each category of pneumothorax has its own risk factors and principles of management. Management principles have shifted from an always interventional approach to increasing consideration of “watchful waiting.” Conservative management in stable patients is encouraged, and if intervention is undertaken, it is often with a smaller-bore chest tube versus needle aspiration drainage without leaving a chest drain in place. Pneumothorax with persistent air leak remains a significant problem; however, surgical and less invasive measures to appear to be effectives. This chapter will review the current well-established and data-driven diagnostic and management practices, as well as compile the existing data for individual interventions in those clinical scenarios to which consensus guidelines do not apply.