The cerebellum has a fine compartment structure, which represents various discrete functions. Evolutionarily old, medial parts of the cerebellum are involved in the adaptive control of various brainstem and spinal cord functions, and long-term depression (LTD) plays a key role in this adaptive control. To extend these views to evolutionarily newer, lateral parts of the cerebellum involved in cerebral cortical functions such as voluntary movement, perception and language, the internal model hypothesis is instrumental. This hypothesis explains not only the characteristic cerebellar symptom, dysmetria, but also a number of otherwise unexplainable phenomena displayed in movements and mental actions.