Multiple Sclerosis (MS) is the most common demyelinating disease affecting the central nervous system of young adults living in western countries. The clinical suspicion of demyelination, as a pathological process, is high when a young adult develops one or more neurological episodes indicating a damage to white matter tracts within the central nervous system, especially when the optic nerve, brainstem, or spinal cord is involved. The patient with episodes disseminating in time, each of which can be attributed to demyelination, requires no investigation prior to establishing the diagnosis of clinically definite MS, if the age of clinical presentation falls between 20 and 50 years, if different anatomical sites within the central nervous system have necessarily been affected on separate occasions, and if the clinical manifestation is typical for MS. MS in Asian populations is characterized by selective and dominant involvement of the optic nerve and spinal cords, as well as some incidence of brainstem lesion and symptoms. About 35~40% of cases of MS in Korea are of this optico-spinal type. Optico-spinal MS (OSMS) generally has a higher female-to-male ratio than conventional MS. OSMS is also characterized by frequent relapses, severe disability, few brain lesions on MRI, and a very lower incidence of oligoclonal bands in CSF. Neuromyelitis Optica (NMO) (Devic syndrome), which causes severe optic neuritis (ON) and longitudinally extensive transverse myelits either with a monophase or relapse-remitting pattern, is rare in Korean. NMO-IgG was reported to be helpful for the diagnosis of early-stage NMO and differential diagnoses of MS.