This article presents endoscopic evaluation of the pleural cavity, or thoracoscopy, an effective diagnostic technique that can be employed to provide additional diagnostic information in cases of intrathoracic disease. The techniques of thoracoscopy are described, and normal and abnormal findings are discussed. Thoracoscopy allows visual examination of the pleural space and surrounding structures without surgical exploration. The stress, expense, morbidity, and mortality of thoracoscopy are far less than those of thoracotomy. Disease for which thoracoscopy has been employed diagnostically include primary and metastatic neoplasia, hilar lymphadenopathy, pericardial effusion, spontaneous pneumothorax, and diaphragmatic hernia. Therapeutically, thoracoscopy has been used for drainage of pericardial effusion.