The autonomic nervous system plays a central role in the maintenance of hemodynamic stability. Dysfunction of this complex regulatory system can lead to the development of loss of consciousness. This article summarizes our current understanding of the role of the autonomic nervous system in maintaining a stable blood pressure and heart rate under normal and abnormal physiologic conditions. The role of baroreceptors, mechanoreceptors, chemoreceptors, vascular reactivity, and the interaction of these sensor systems with the central nervous system as a whole are reviewed. Current concepts related to the mechanisms of unexplained syncope and the "state-of-the-art" diagnostic and treatment options are also discussed.