Platelets play a pivotal role in hemostasis by forming the initial hemostatic plug and augmenting the formation of the more permanent fibrin thrombus. Thrombocytopenia may lead to bleeding, which can be treated with therapeutic platelet transfusions or prevented with prophylactic transfusions. While many advances have been made in platelet transfusion technology, problems remain. These include the short storage life of liquid-stored platelets and the frequent development of alloantibodies (the refractory state) in patients receiving a number of transfusions. Significant progress, however, is being made in both areas.