Most cases of hemothorax are related to blunt or penetrating chest trauma. Criteria for surgical intervention for initial hemothorax are well defined. Appropriate management of retained hemothorax following initial trauma management is critical, and the best approach remains controversial. Spontaneous hemothorax is much less common and results from a variety of pathologic processes. This article reviews the etiology, diagnosis, and treatment of traumatic and spontaneous hemothorax in modern practice.