For decades, cerebellar research has been guided by the central hypothesis that plasticity at synapses in the cerebellar cortex mediates motor learning. This hypothesis has been challenged especially strongly in recent years by data that contradict the original theories of cerebellar motor learning. These data form the basis for strong arguments to the contrary and have inspired new, non-motor theories of cerebellar function. We consider key data that are contrary to the motor learning hypothesis and develop the argument that both old and new data are best explained by extending, rather than rejecting, the basic tenets of the original motor learning theories of cerebellar function. NEUROSCIENTIST 3:303–313, 1997
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