Publisher Summary This chapter presents an overview of acute optic neuritis. Optic neuritis is a common cause of acute visual loss in early adult life. In the large majority of cases, the prognosis for vision is excellent even without treatment, but precise diagnosis is nevertheless important for two reasons. First, optic neuritis may be simulated by other causes of visual failure (such as compression of the optic nerve) that require specific treatment if permanent visual loss is to be avoided and second, the recognition of a present or past attack of optic neuritis is often critical in the diagnosis of multiple sclerosis (MS). The hallmark of optic neuritis is a plaque of demyelination, that is, a focal zone of myelin loss in which the continuity of the axons is preserved. In optic neuritis occurring in the course of MS, the mean length of the plaque is approximately 1 cm. This chapter discusses the clinical features, diagnosis, and management of optic neuritis. It also describes advances in the techniques for detecting subclinical damage to the optic nerves.