Cardiopulmonary bypass (CPB) is a nonphysiologic state that has many detrimental effects on a patient's hemostatic integrity. Exposure to the extracorporeal circuit and subsequent activation of the coagulation and fibrinolytic systems are factors that contribute to morbidity and mortality in cardiac surgical patients. These effects can be prevented in part or appropriately treated if practitioners understand the basic mecha nisms. This article reviews the effects of CPB on platelet function, the relationship of platelet function to post operative bleeding, the monitors available to measure platelet function, and the impact of antiplatelet therapy on bleeding in cardiac surgery.