Multiple sclerosis (MS) is considered a prototypic autoimmune disease of the central nervous system (CNS). A complex genetic background with the HLA-DR15 haplotype as the main genetic risk factor and over 100 mostly immune-related minor risk alleles as well as several environmental factors contribute to the etiology of MS. With respect to pathomechanisms, autoimmune inflammation in early MS is primarily mediated by adaptive immune responses and involves autoreactive T cells, B cells, and antibodies, while the later, chronic stages of MS are characterized by a compartmentalized immune response in the CNS with activated microglia and macrophages. A host of immune cells and mediators can contribute to the autoimmune process, but CNS-related factors such as localization of lesions, vulnerability of oligodendrocytes, neurons/axons, and secondary metabolic changes all play a role in the heterogeneous expression of the disease, including different pathologic lesion patterns, neuroimaging findings, disease courses, and severity and response to treatment.