The major event in relapsing remitting multiple sclerosis (MS) is the acute relapse. The majority of patients with MS present with such an event. Most early relapses are followed by complete or near complete recovery, however partial recovery or significant disability can result from an individual relapse. Study of relapses in vivo is therefore desirable. Magnetic resonance imaging (MRI) is very sensitive at detecting MS lesions. Unfortunately, in the brain and spinal cord, it has been difficult to reliably identify the lesion that is responsible for the symptoms of an individual relapse and most new brain lesions which are apparent on MRI are clinically silent. Optic neuritis provides an attractive model to study the effects of relapses in MS. Optic neuritis is a frequent manifestation of MS and has been regarded as a forme fruste of the disease. The natural history of acute optic neuritis mirrors that of an acute MS relapse elsewhere in the central nervous system (CNS) and the response to corticosteroid therapy is the same. Visual function can be measured accurately in a rater-independent fashion and it is possible to measure the latency and amplitude of the visual evoked potential (VEP) which gives information about the integrity of the visual conducting pathways. Developments in imaging technology and application now means that, potentially more pathologically specific imaging sequences can be applied to the study of the optic nerves. This thesis will investigate the application of these new imaging techniques, along with clinical and VEP measures, to the study of optic neuritis in order to gain new insights into the effect of individual lesions on structure and function in the CNS in the short- and long-term.