Recent anatomic and physiologic studies have shed new light on the functional organization of the basal ganglia and their role in movement. The basal ganglia receive topographically organized input from the entire neocortex. Influences from sensorimotor and "association" cortices appear to remain segregated in the basal ganglia. The concept of segregated parallel subcortical loops subserving "motor" and "complex" functions is discussed. Recent neurophysiologic studies in behaving primates suggest that basal ganglia output plays a role in controlling the direction and amplitude of movement but is not primarily involved in the initiation of limb movement or selection of specific muscles. These studies are generally consistent with data from patients with Parkinson's disease, which likewise indicates a deficit in the programming of movement amplitude in step-tracking tasks, with little or no change in reaction-time or pattern of muscular activity