Transfusion of packed red blood cells in critically injured patients has been a lifesaving (although not completely benign) intervention for decades. The traumatically injured brain has been thought to be particularly susceptible to injury from anemia, due to the well-documented association of worsening mortality and functional outcome in the presence of hypotension and hypoxia, as well as the known vulnerability of many neuronal populations to ischemia. Red blood cell transfusion has been used in traumatic brain injury (TBI) to prevent cerebral ischemia by maximizing the oxygencarrying capacity of blood that is otherwise decreased by blood loss and dilution with crystalloid fluid replacement during resuscitation. Although many practitioners have commonly utilized hemoglobin (Hgb) or hematocrit thresholds for transfusion in these patients, the rationale for this practice has largely been centered on older studies in general critical care populations and animal evidence. Furthermore, in addition to an ideal " target " Hgb, many other questions remain about this clinical practice, such as the optimal duration of maintaining a specific Hgb level, and the ultimate effects of transfusion on neurological and functional outcome.