Circulating blood platelets survey the integrity of the vascular wall and respond after tissue trauma to control hemorrhage. During the initial response to vascular injury platelets adhere to injured surfaces and aggregate to form a platelet plug or thrombi. However, the complete arrest of hemorrhage depends not only upon platelets but other factors, as well, collectively referred to as hemostasis. For normal hemostasis to occur platelets along with the blood coagulation cascade act cooperatively to arrest blood flow. Nevertheless, platelets within the blood clot are of paramount importance as evidenced by the critical bleeding occurring with a reduced platelet count, a condition referred to as thrombocytopenia. Moreover, the same processes occurring under normal circumstances to prevent hemorrhage may also interfere with blood flow in atherosclerotic arteries and cause life-threatening situations, such as myocardial infarction, the leading cause of death in developed countries. Thus, normal platelet function involves maintaining a balance between preventing excessive blood loss and allowing essential blood flow to critical tissues and organs. (c) 2001 Prous Science. All rights reserved.