Recordings from the cerebellum under behavioral conditions that cause learning in the vestibulo-ocular reflex (VOR) constrain the cellular mechanisms that could mediate learning. Analysis of the complex-spike responses of Purkinje cells demonstrates a mismatch between the properties of cerebellar long-term depression (LTD) in vitro and the signals available to guide learning in vivo. To resolve this mismatch, it may be necessary to assume that there are multiple cellular mechanisms of VOR learning, including both depression and potentiation.