Multiple sclerosis (MS) is an autoimmune disease triggered by environmental factors that act on a genetically susceptible host. It features three clinical stages: a pre-clinical stage detectable only by MRI; a relapsing-remitting (RRMS) stage characterized by episodes of neurologic dysfunction followed by resolution; and a progressive stage, which usually evolves from the relapsing stage. Advances in our understanding of the immune mechanisms that contribute to MS have led to more than ten FDA-approved immunotherapeutic drugs that target effector T cells, regulatory cells, B cells, and cell trafficking into the nervous system. However, most drugs for relapsing MS are not effective in treating progressive disease. Progressive MS features a compartmentalized immune response in the central nervous system, involving microglia cells and astrocytes, as well as immune-independent processes that drive axonal dysfunction. Major challenges for MS research involve understanding the mechanisms of disease progression, developing treatment for progressive MS, and determining the degree to which progressive disease can be prevented by early treatment. Key priorities for MS research include developing biomarkers, personalized medicine and advanced imaging, and a better understanding of the microbiome. With a better understanding of the genetic and epidemiological aspects of this disease, approaches to prevent MS are now also being considered. Baecher-Allan et al. describe their latest understanding of the immunopathogenesis of MS, which is triggered by both genetic and environmental factors. Delineation of the immune process in MS has led to multiple disease-modifying therapies that are in wide use.