The decision to initiate transfusion is complex. There are many risks to transfusion (see Section VIII), and transfusion is ultimately indicated when the potential benefits outweigh these risks. In most civilian settings, transfusion therapy utilizes blood components rather than whole blood. The benefits of blood component transfusion stem from the correction of conditions that result in decreased production; increased utilization, destruction, or loss; or dysfunction of a specific blood component (usually red blood cells, platelets, or coagulation factors). Although many attempts at creating universal transfusion protocols have been introduced, few have successfully demonstrated definitive thresholds or clear superiority. The aim of this chapter is to provide the background to help clinicians make these decisions on an individual basis. A condensed summary of approaches to perioperative transfusion therapy and decision-making may be found in Table 36.1.