The vestibulo-ocular reflex, because of its close relationship with the cerebellum and its marked adaptiveness, has become a model system for studying the functions of the cerebellum. It has been hypothesized that an evolutionarily old part of the cerebellum, the flocculus, forms a modifiable accessory pathway for the vestibulo-ocular reflex arc for adaptive control, and that the modification is due to the synaptic plasticity induced by retinal errors conveyed by a unique structure of the cerebellum, the climbing fibers. The flocculus hypothesis has been supported by several lines of evidence, including lesioning or functionally impairing the flocculus and recording the activity of flocculus Purkinje cells, and, more recently, from pharmacologically or genetically inhibited synaptic plasticity, which produces long-term depression. There has also been debate on a possible site for memory retention in vestibulo-ocular-reflex adaptation, and about the signal content in flocculus Purkinje cells. This article reviews recent studies on the learning mechanisms of the cerebellum that underlie the adaptation of the vestibulo-ocular reflex.