The inferior olivary nucleus is the sole source of an entire afferent system to the cerebellum, the climbing-fiber system. Inferior olivary neurons are very sensitive to the appropriate sensory stimuli, such as light contact to the paw. Yet, when animals move about, olivary cells show little change in discharge rate. Apparently some mechanism prevents the cells from discharging to stimuli generated by the animal's own movement. The inferior olive receives a massive inhibitory input from small cells in the cerebellar and vestibular nuclei. This article reviews the results from several experiments that suggest that the inferior olive is specifically targeted by inhibitory inputs that prevent responses to stimuli resulting from self-produced movement. Oscarsson proposed that the inferior olive provides the cerebellum with information about errors of motor performance and about spinal reflexes. We argue that it is unlikely that the inferior olive provides information about movement errors, although the olive may signal the occurrence of sensory events that are likely to elicit reflex movements. Another popular theory of climbing-fiber action argues that the climbing fibers play a role in altering the strength of the parallel fiber-Purkinje cell synapse. The cerebellum is important for the formation of classically conditioned responses, and input generated by the unconditioned stimulus does provide effective stimulation of olivary neurons. Although the olive does not generate the unconditioned response, it may provide the cerebellum with information necessary for the formation of conditioned responses.