Considerable effort has gone into determining what the trigger should be for the transfusion of platelets and how many platelets should be transfused. Historically, the platelet transfusion trigger for patients with chemotherapy-induced thrombocytopenia was 20 × 10⁹/μL. However, several recent large studies have suggested that 10 × 10⁹/μL is acceptable. A large retrospective study has shown that the most significant predictor of bleeding in thrombocytopenic patients is not the platelet count but a history of bleeding in the previous 5 days. These data suggest that attention should be focused on providing aggressive platelet therapy for active bleeding rather than on prophylactically transfusing platelets. Such an approach may not be appropriate in trauma patients, in whom hypothermia, acidosis, and other factors may affect the function of the patient’s own platelets and the platelets that are transfused. Specific situations requiring platelet transfusions in thrombocytopenic patients include the presence of bleeding and the need to perform invasive procedures such as lumbar puncture, bronchoscopy, and surgery.