DeLong and Wichmann point out that the anatomy and function of the basal ganglia and their role in motor and nonmotor disorders have undergone major revisions over the past decades. They review the evidence that the basal ganglia are now appreciated as components of parallel, re-entrant cortico-subcortical circuits, which take origin from individual cortical areas, traverse the basal ganglia and thalamus, and terminate in their respective areas of origination in the frontal lobe. This view underlies current thinking about the physiology of normal basal ganglia function and in abnormal circuit functions underlying Parkinson disease and other common movement disorders. They emphasize that circuit disorders is an appropriate designation to describe diseases of the basal ganglia resulting from pathologic disturbances in neuronal activity throughout specific cortico-subcortical loops. It is a clear and compelling explanation and review of the functions of the basal ganglia in health and disease written by the principal investigators.