Four organizational levels of the basal ganglia that could be particularly determinant in terms of functional properties are reviewed: (1) macroscopic anatomy, which is characterized by a dramatic decrease of cerebral tissue volume from the cerebral cortex to the deepest portions of the basal ganglia; (2) connectivity, which consists of both complex loops and a partition into three territories, sensorimotor, associative, and limbic (which process motor, cognitive, and emotional information, respectively); (3) neuronal morphology, characterized by a dramatic numeric and geometric convergence of striatal neurons onto pallidonigral neurons; and (4) dopaminergic innervation of the basal ganglia, which is organized as a dual system that is supposed to have opposite effects on the activity of the system. Current models of the basal ganglia are discussed.