A thoracic empyema is more than a collection of pus in the pleural space; it is an actual abscess. The inflamed pleurae form its walls, and it is commonly divided by bands and adhesions into subspaces with little apparent intercommunication. It usually develops by lymphatic bacterial drainage into an initially sterile pleural effusion, by direct extension from the lung itself, by rupture of a subpleural abscess, by septic embolization, or by traumatic or iatrogenic contamination. For rapid and complete healing the abscess must be completely evacuated. Antibiotics must be used with pinpoint accuracy, or a chronically ill patient will come to harbor an organism resistant to all usual antibiotics. These principles are illustrated by ten case reports.