Multiple sclerosis (MS) is a disease of unknown origin and for which there is no specific diagnostic test. The diagnosis of MS is always the result of a more or less simple procedure, depending on the cases, and it remains uncertain until evidence of anatomo-pathological signs have been brought forward. An almost unanimous consensus does exist regarding several points. In the presence of suggestive or, at least, compatible neurological signs and in the absence of any alternative diagnosis according to a neurologist expert in MS, the diagnosis of MS is based on three criteria: the evidence of at least two different lesions in the white matter of the central nervous system, which is called the "space dissemination" criterion; the evidence of at least two different episodes in the disease course, which is called "time dissemination"; the evidence of a chronic inflammation of the central nervous system revealed through the analysis of the cerebrospinal fluid, i.e. the "infammatory" criterion. The observation of one or several of these criteria allows to establish the diagnosis of MS with more or less certainly, likely to be reassessed according to the subsequent course of the disease. There is a wide consensus today with Poser's classification (Poser et al., 1983) which combines the three criteria and distinguishes five different categories. According to Poser et al., space dissemination may be proved on clinical examination, but also with the brain MRI and, failing that, with the evoked potentials. Time dissemination is based only on clinical signs and the inflammatory criterion is as significant as the two other criteria. This classification has just been revised by an expert committee (McDonald et al., 2001) who wish to simplify it into two different categories (MS vs possible MS). It has been suggested that time dissemination should be proven by the observation of MRI signs three months at least after the previous clinical episode or the previous MRI. It has been suggested also to use the inflammatory criterion in second position in replacement of the space criterion when the latter is missing on the clinical and paraclinical levels. Time will say whether this new classification will replace the previous one.