This chapter reviews the most influential ideas on basal ganglia (BG) function. There are parallel cortico-BG-thalamocortical loops, defined by distinct functional territories in the cortex and their corresponding BG targets. These reentrant circuits are believed to be basic units of function at the level of behavior. According to the rate model, because the BG output is inhibitory and tonically active, at rest it serves to prevent action initiation. Too much output leads to behavioral suppression, and not enough output leads to uncontrollable behavior. According to the focused selection model, the direct pathway achieves action selection using disinhibition, whereas the indirect pathway serves to suppress competing behaviors. According to the central selection model, the BG act as a central selector that receive bids from different motivational goals that must compete for access to effectors. The BG are also thought to implement reinforcement learning models, in which a prediction error can change the probability of action selection in a given state. Because conventional models all assume that behavior is categorical, they fail to account for the relationship between continuous behavioral variables and BG activity, or the key symptoms following damage to the BG, such as bradykinesia, rigidity, and asymmetric postures.