Massive uncontrolled blood loss can occur under a variety of conditions including trauma, as a complication of childbirth or surgery, ruptured ulcers, clotting disorders, and hemorrhagic fevers. Across the continuum of hemorrhage, loss of blood volume is a significant challenge to the maintenance of cerebral perfusion. During the initial stages of hemorrhage, reflex mechanisms are activated to protect cerebral perfusion, but persistent blood loss will eventually reduce global cerebral blood flow and the delivery of metabolic substrates, leading to generalized cerebral ischemia, hypoxia, and ultimately, neuronal cell death. Cerebral blood flow is controlled by various regulatory mechanisms, including prevailing arterial pressure, intracranial pressure, arterial blood gases, neural activity, and metabolic demand. Hemorrhage represents a unique physiological stress to the brain, as it influences each of these regulatory mechanisms, resulting in complex interplay that ultimately challenges the ability of the brain to maintain adequate perfusion. Early studies of actual hemorrhage in humans employed blood loss protocols up to 1000 mL, but did not include any measurements of cerebral blood flow. As ethical considerations necessarily constrain the use of human volunteers for massive blood loss studies that induce irreversible shock, most of what is known about cerebral blood-flow responses to hemorrhage has been determined from animal models. Limitations of species differences regarding regulatory mechanisms, anatomy, and the effect of anesthesia, however, must be considered. Advances in monitoring technologies, and a recent renewed interest in understanding cerebral blood-flow regulation in humans, however, is rapidly accelerating knowledge in this field.