An increasing number of patients referred for neuroimaging studies unrelated to multiple sclerosis (MS) arc found to have incidental lesions suggestive of MS in their nervous systems but Many such patients do not develop clinical symptoms and signs and remain as "subclinical MS" (SCMS). MRI and cerebrospinal fluid (CSF) studies in monozygotic twins and siblings of MS patients, as well as necropsy studies in asymptomatic persons also reveal findings consistent with SCMS. Clinically isolated syndromes (CIS), acute disseminated encephalomyelitis (ADEM), benign MS, relapsing-remitting MS, primary and secondary progressive MS, optico-spinal MS, Balo's and Marburg's diseases, are considered different forms of MS by many because of their heterogeneous clinical, imaging and pathological features. Studies of disease susceptibility, type, course and severity have given different results in different populations, consistent with "genetic heterogeneity". The various forms of the disease may change from one to another, and their clinical and imaging features became similar after a number of years. The current data Support the concept that MS, whether it is a single disease or a group of disorders, is an immune-mediated disease of the CNS, with both inflammatory and degenerative features with available therapies being only partially and temporarily effective for the inflammatory phase of the disease. Making the diagnosis of MS may not be an absolute indication for early treatment with disease modifying agents. The use of new technology Such as microarray and other techniques in diagnosing and understanding the MS spectrum may make it possible to recognize the different molecular subtypes of these diseases and develop better therapies. (c) 2005 Published by Elsevier B.V.