The ability to maintain balance is essential to nearly all activities associated with daily living. The balance system enables us to sense where we are in space and to maintain our posture and equilibrium while we are still and while we are in movement. We often do not consider balance as a "sense" in the same way that we think of seeing, hearing, tasting, smelling, and so on, because balance is usually an automatic, unconscious process. However, impairment of the balance system can be a major disabling condition for those who are affected. The control of balance requires the integration of information from multiple sensory and motor systems by the central nervous system (CNS). Balance receptors in the inner ear (the vestibular system) provide information to the CNS about head and body movements. The eyes (visual system) provide input regarding the body's orientation within the environment and about motion within the environment. The position and motion sensors of the muscles and joints, and the touch receptors of the extremities (proprioceptive system) send signals regarding bodily position, particularly in relation to the support surface. The CNS integrates all this data, determines the body's spatial orientation, and sends appropriate neural messages to the motor system to activate movements that will maintain equilibrium. Because the balance system is so complex, it can be impaired by a large number of disease processes affecting any of the multiple sensory inputs, neural processing centers, or motor outputs. These include (but are not limited to) infections of the inner ear, head injury, drug or environmental toxicity, cerebrovascular insults, degenerative changes due to aging or illness, and autoimmune reactions. Balance disorders can result in a wide variety of symptoms, ranging from a generalized feeling of disorientation and disequilibrium to acute vertigo (i.e., the sensation of motion, particularly spinning of the body or the environment). Because balance is normally an unconscious process, patients often have difficulty articulating their symptoms; and physicians can have difficulty determining the exact cause of the problem. Impairments of balance are common and debilitating conditions. People with chronic balance disorders are significantly disabled in many day-to-day functions, particularly those that require stabilizing the body during weight-shifting, bending, or rapid head motion. Examples of such tasks include getting in and out of bed, taking a bath or shower, climbing stairs, and reaching for objects in high or low cabinets. Changes in body posture that require shifting one's weight or moving the head are particularly problematic, and can induce vertigo. In addition, those who have balance impairments have more difficulty than normal individuals doing tasks that involve spatial perception and movement, and often report bumping into walls, walking off a sidewalk, or veering into another lane while driving. Balance disorders restrict an individual's normal motor activities, thereby limiting one's sense of independence and adversely affecting the quality of life. The prevalence of balance disorders in the overall population of the United States is unknown. It is estimated that at least half of the overall population of the United States are affected by a balance or vestibular disorder sometime during life. Several groups are particularly at risk. The National Institute on Deafness and Other Communication Disorders (NIDCD) estimates that up to two-thirds of children with acquired deafness have severe vestibular deficits. In addition, individuals who have sustained head injuries are likely to suffer from problems with balance and dizziness. Healthy individuals who are exposed to unusual motion and gravity environments, such as divers, high-speed pilots, and astronauts, are another affected group.