Multiple sclerosis (MS) is a central nervous system disorder, characterized by mononuclear cell inflammation, demyelination and often with extensive axonal injury. It was first described neuropathologically in the late 1800s. MS has an interesting geographical epidemiology, with a higher rate at latitudes further from the equator in both directions. Women outnumber males by about 2:1; this ratio has been increasing in recent years. Genome wide association studies have thus far identified over 50 genetic susceptibility loci, and these are rapidly expanding. Several environmental risk factors have been identified, including low serum vitamin D levels, exposure to Epstein-Barr virus and cigarette smoking. MS displays a heterogeneous disease course; most patients with the disease begin with a relapsing-remitting course, but often eventually develop steady disability progression. A small percentage of MS patients have a progressive course without clinical relapses. Several treatments are now available to decrease relapse rate and slow the accumulation of disability in patients with relapsing MS, but there is currently no effective treatment to slow the progressive forms of MS.