Basic knowledge regarding platelet physiology has been reviewed and related to clinical situations where platelet function is critically important. Platelet function tests such as aggregation and survival studies, which are research tools now, may soon be available to aid in the diagnosis and management of clinical bleeding problems. A growing understanding of drug-platelet interactions is allowing manipulation of platelets and the clotting system to improve the proganoses for patients who have thromboembolic diseases. Advances in platelet storage and transfusion techniques can save patients who previously might have bled to death. Proper application of the new developments in cardiopulmonary bypass and hemodilution, with attention to platelet and coagulation factor preservation, will contribute to securing hemostasis and conserving blood products.