The decision to transfuse is complicated. There are many risks to transfusion, and transfusion is ultimately indicated when the potential benefits outweigh these risks. In most civilian settings, transfusion therapy utilized 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 (red 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. Our aim in this chapter is to provide the background to help make these decisions on an individual basis.