This chapter discusses the etiology, diagnosis, medical management and treatment of pyothorax. The etiology of pyothorax tends to be different in dogs when compared with cats. The clinical signs for a dog or cat with pyothorax are variable and can be delayed for weeks to months after the inciting incident. Stabilization of the patient with pyothorax presenting in respiratory distress should include oxygen supplementation, minimal handling and stress, thoracocentesis, and intravenous catheter placement. Thoracic ultrasonography can be used to confirm the diagnosis of pleural effusion and determine the best site for thoracocentesis. Drainage of pleural effusion is one of the mainstays of therapy for patients with pyothorax. Surgical treatment of pyothorax requires a complete exploration of the thoracic cavity. Options for surgical therapy include median sternotomy, lateral thoracotomy, and thoracoscopy (video‐assisted thoracic surgery, VATS). Cardiovascular monitoring is important in the postoperative patient.