Multiple sclerosis (MS) is a chronic inflammatory demyelinating disease that affects the central nervous system. MS is more prevalent in women and is estimated to affect some 2.3 million people across the world. There is unequivocal genetic susceptibility in MS. The most consistent genetic determinant identified is the major histocompatibility complex (MHC). The haplotypes more strongly related to susceptibility and protection for MS are HLA-DR2 and HLA-DR11, respectively. Some genes outside the MHC, such as IL2RA, IL7R and TNFRSF1A, have also been related to MS. There is a latitudinal gradient of MS prevalence, probably due to environmental factors on the genetic susceptibility. The most important MS risk factors are seropositivity against Epstein–Barr virus, infectious mononucleosis, and smoking. Other factors such as vitamin D or parasitic infections require further investigation. The clinical manifestations of relapsing forms of MS in initial stages are related to demyelination of the susceptible structures such as the optic nerves or spinal cord. In established MS, the clinical symptoms are related to the multisystemic affectation and neurodegeneration such as cognitive impairment or sphincter disorders. An unmet need exists for highly effective medications with low risk for deep immunosuppression and for the symptomatic relief of MS.