Whereas there are general guidelines for acceptable transfusion therapy, optimal transfusion therapy has not been determined for most clinical settings. Recent research has focused on controlled studies of red cell transfusion in specific clinical settings. Better determinations of oxygen delivery and consumption are needed to guide clinicians in determining whether transfusion is justified for patients during the perioperative period, those with coronary artery disease, and those in intensive care units. For sickle cell disease, the role of transfusion for acute complications can be life saving; however, the role of chronic transfusion regimens awaits further research into efficacy. Finally, whereas criteria for the prophylactic transfusion of platelets in hematologic diseases are well described, relatively little information is available on the value of platelet transfusion where the absolute count is less than 100,000 but greater than 50,000. The value of fresh frozen plasma components, both standard and sterilized, also requires elucidation.