A general functional model of the cerebellum is proposed based on recent anatomical physiological findings. A cerebellar microcomplex composed of a cortical microzone and a small cell group in a cerebellar or vestibular nucleus acts as an adaptive controller based on the synaptic plasticity of the long-term depression type in Purkinje cells, which is effected by control error signals of climbing fibers. Microcomplexes will be inserted in reflex arcs, command systems of voluntary motor control, and probably even cortical systems performing certain mental activities, providing adaptive-learning capabilities to these systems.