Postoperative bleeding is caused by alterations in the normal process of hemostasis or by a surgical problem. The role of platelets, coagulation proteins, and fibrinolysis in the control of postoperative bleeding is re viewed, as are preoperative and intraoperative factors that interfere with these normal processes. Current treatment of postoperative bleeding is discussed, including the use of positive end-expiratory pressure, control of hypertension, medication, and blood products.