The course of multiple sclerosis (MS) results from the interplay of two clinical processes, relapses and progression. Three main clinical forms may be identified as relapsing-remitting, secondary progressive and primary progressive. The disease appears on average at the age of 30 and follows a remitting or a progressive course in 85% and 15% of the cases, respectively. Initial symptoms are related to an isolated or combined disturbance(s) of the long tracts of the central nervous system, of the brainstem, or of the optic nerve in 70%, 20% and 25% of the cases, respectively. The relapse rate is one relapse on average every other year. After an exacerbating-remitting onset of MS, secondary progression appears after 19 years on average. The median time to reach the landmarks of irreversible disability are 8 years regarding limitation of ambulation, 20 years for walking with a stick, and 30 years for wheel-chair dependency. In fact, MS prognosis is highly variable depending on individuals. All the intermediate types do exist between malignant forms, possibly lethal, and benign forms (circa 30% of the cases) that allow normal daily life. The clinical and paraclinical predictive factors identified so far have been acknowledged statistically. They provide little help however, if any, when an individual is concerned. Recent research has showed that relapses have only a marginal effect, relatively to progression, on the accumulation of irreversible neurological disability. Furthermore, the age when reaching the landmarks of irreversible disability is essentially similar whatever the mode of onset of MS, be it exacerbating-remitting or progressive. In spite of the outstanding clinical polymorphism of MS, a unifying concept of the disease ("complexity vs heterogeneity") can be put forward.