A new theory of cerebellarfunctionisproposed. It is suggested that the cerebellum works according to a hitherto unknown principle of motorcontrol. A simple learning process in the cerebellum, the 'coupling operation', seems powerfule nough to explain the role of this brain part in training and performance of complex motor skills. The coupling operation greatly enhances the capability to perform fast, accurate and highly coordinated motions. These motionscan be executed by many different paths, while being welladapted to changing intentions and situations. However, the cerebellar 'coupling machine' does not control movements by itself; rather, it is a supportive device. In the case of skilled motions, forinstance, the controlisalwaysleftto the forebrain. A network model of the cerebellum demonstrates how the coupling operation might be carried out in cellular detail in a biologically plausible way. According to the theory, the cerebellum only influences complex voluntary movements, and not simple ones. This theory explains a great deal of physiological, anatomical and clinical data, producing a coherent picture.