Multiple sclerosis (MS) is the most common disease of the central nervous system that causes permanent disability in young adults. Based on strong circumstantial evidence, MS is considered to be putative autoimmune disorder, but much remains to be understood about the etiology and clinical onset of the disease. It seems unlikely that MS results from a single causative event, but rather is the result of genetic and environmental factors and the interactions thereof. This articlediscusses the epidemiology of MS.