The basal ganglia (BG) have been implicated in motor control, language, attention, working memory, and procedural learning. Willis, who was the first to describe these subcortical nuclei clearly, believed that they are critical for sensation, movement, and imagination. Others argued that the BG are merely a relic in the mammalian brain, its function largely replaced by the cerebral cortex. More recent work suggests that BG evolved together with the cerebral cortex, together forming a basic cerebral anatomical motif that has been evolutionarily conserved. Yet despite centuries of research, the functions of these nuclei remain a subject of contention. What is missing in current models of BG function is an adequate model of behavior. The standard model of BG function assumes that the BG select categorical actions. However, recent work has begun to question this assumption. At the level of integrative function, the challenge is to determine the computational functions of each BG circuit and how different cortico-BG circuits are coordinated to generate behavior.