Glatiramer acetate (GA, Copaxone [Teva Pharmaceuticals, Kansas City, MO], formerly known as copolymer-1) and interferon- (IFN)-β are both used for the immunomodulatory treatment of multiple sclerosis, but they act in different ways. Four major mechanisms of GA have been identified: 1) competition with myelin-basic protein (MBP) for binding to major histocompatibility complex (MHC) molecules; 2) competition of GA/MHC with MBP/MHC for binding to the T-cell receptor; 3) partial activation and tolerance induction of MBP-specific T cells (action as an altered peptide ligand); and 4) induction of GA-reactive T-helper 2- (TH2)-like regulatory cells. Of these four mechanisms, 1 and 2 presumably occur only in vitro and are therefore irrelevant for the in vivo effects of GA. In contrast, mechanisms 3 and 4 could occur in vivo and both could contribute to the clinical effects of GA.