Multiple sclerosis is an inflammatory demyelinating disease of the central nervous system in which destruction of myelin and nerve axons has been shown to be mediated by immune mechanisms. Although the focus of research has been traditionally on T cells as key mediators of the immunopathology, more recent efforts at understanding this complex disorder have been directed increasingly at other cellular and humoral elements of the immune response. This review is a reappraisal of the crucial role of T cells, in particular the CD4+ helper T-cell subset, in multiple sclerosis. Recent evidence is discussed underlining the predominant contribution of T-cell-associated genes to the genome-wide association study results of multiple sclerosis susceptibility, the loss of T-cell quiescence in the conversion from clinically isolated syndrome to clinically definite multiple sclerosis, and the fact that T cells represent the main target of effective immunomodulatory and immunosuppressive treatments in multiple sclerosis.