The term “optic neuritis” compromises all diseases of the optic nerve that cause primary demyelination and usually manifest themselves as a sudden visual field defect or total loss of vision in one or both eyes. As in man, the cause of optic neuritis is often difficult to determine in the living animal. Neurologic examination, cerebrospinal fluid analysis, and laboratory tests can be normal. Optic neuritis affects dogs far more frequently than other animal species. This article examines the causes of immune-mediated optic neuritis and etiologic differentials in dogs and cats, with comparisons to human beings and laboratory animal models.