Empirical evidence suggests that the human cerebellum plays a role in motor learning processes such as conditioning, habituation and adaptation. In addition, the cerebellum is said to be involved in the learning of complex skills as shown by many functional imaging and behavioral studies. However, the exact function of the cerebellum in implicit motor learning is still matter of debate. While it seems that an intact cerebellar cortex is necessary for visuo-motor adaptation, it remains unclear how adaptation to an unknown force-field is mediated by the cerebellum. Recent imaging studies in which subjects performed goal-directed arm movements in an external force-field revealed an activation of the ipsilateral cerebellar cortex during the initial adaptation. A study of our group tested how humans with cerebellar degeneration adapted goal-directed arm movements to an unknown external force-field and whether learning could be generalized to untrained regions in the workspace and if any learning could be retained. After removal of the force-field, patients with cerebellar degeneration showed little or no learning-related after-effects indicating that repeated force-field exposure never led to successful force compensation. Moreover, the generalization of learned movements to targets outside the learned workspace was prevented by the cerebellar degeneration. These and other findings suggest that cerebellar dysfunction prevents the formation of an internal representation of the limb dynamics.