Pneumothorax is a pathological condition in which air accumulates within the thoracic cavity. Pneumothorax affects animals without sex or age predilections; however, it has been suggested that the Siberian husky breed of dog has a predisposition for spontaneous pneumothorax. Pneumothorax occurs as the result of trauma or underlying disease and can present a clinical challenge with regard to diagnostic and therapeutic techniques. Topics reviewed include normal lung physiology; the pathogenesis, diagnosis, treatment, complications, and prognosis of pneumothorax; and current techniques in animals and humans.