The pathogenesis of multiple sclerosis is still poorly understood. Clearly, exogenous factors and genetic susceptibility combine to induce the disease, but studies on twins have shown that the genetic part is of minor importance. The immunological abnormalities observed do not fit easily into a coherent system. Derepression of antibody production in the central nervous system has been postulated and ascribed to failure of suppressor T-lymphocytes, but this is an over-simplification and the possibility that immunological abnormalities merely reflect a genetic predisposition cannot be dismissed. For many years, a viral origin has been suspected, yet the measles virus, at the top of the black list, must probably be discharged. Although the auto-immune theory remains attractive, the target antigen is still unknown in spite of numerous studies, and the signs of auto-immunity found in multiple sclerosis might well result from destruction of the nervous tissue instead of being the cause of the disease.