Pneumothorax occurs when air enters the pleural space, disrupting the normal negative intrapleural pressure that maintains lung inflation. Primary spontaneous pneumothorax (PSP) occurs without underlying lung disease, typically in tall, thin young males from rupture of apical subpleural blebs. Secondary spontaneous pneumothorax (SSP) occurs in patients with underlying lung disease (COPD most common, followed by asthma, CF, Pneumocystis pneumonia, Langerhans cell histiocytosis). Traumatic pneumothorax results from penetrating or blunt chest injury. Tension pneumothorax is a life-threatening emergency: a one-way valve mechanism allows air to enter but not exit the pleural space, causing progressive accumulation, mediastinal shift, and compression of the contralateral lung and great vessels, leading to obstructive shock. Needle decompression at the 2nd intercostal space, midclavicular line (or 4th-5th ICS, anterior axillary line) converts tension to simple pneumothorax as a temporizing measure before chest tube placement.