Trapped lung is one of the outcomes of fibrinous or granulomatous pleuritis and is a cause of chronic, benign, unilateral pleural effusion. It is characterized by inability of the lung to expand and fill the thoracic cavity due to a restricting fibrous visceral pleural peel. The resulting chronic pleural space is fluid filled, and the persistence of the fluid is solely due to hydrostatic equilibrium. Historically recognized as a complication of therapeutic pneumothorax for treatment of tuberculosis, it is today most commonly a consequence of inadequately treated parapneumonic effusion, but it is also associated with cardiac surgery, chest trauma, and other inflammatory processes involving the pleura. The diagnosis requires documentation of chronicity and stability and the absence of an active inflammatory or malignant pleural process, bronchial obstruction, or severe underlying lung disease. Findings supporting the diagnosis are an initial negative pleural liquid pressure, increased pleural space elastance, and the demonstration of a pleural peel. Confirmation of the diagnosis requires successful surgical decortication, which is the only available therapy. In the asymptomatic patient, decortication is not indicated and observation is warranted.