Natural killer T (NKT) cells are a subset of lymphocytes that express receptors characteristic of conventional T cells together with receptors typically found on natural killer cells. A key feature of NKT cells is the expression of a semi-invariant T cell receptor that is specific for glycolipid antigens presented by the unusual major histocompatibility complex class I-like molecule CD1d. While their precise immunological functions remain unknown, NKT cells have been implicated in the regulation of adaptive immune responses, including those directed against autoantigens. These findings raise the possibility that specific stimulation of NKT cells may be exploited for therapeutic purposes. A number of laboratories have tested this hypothesis, utilizing the sea sponge-derived agent alpha-galactosylceramide (alpha-GalCer), a specific agonist of NKT cells. Administration of alpha-GalCer to mice results in potent activation of NKT cells, rapid and robust cytokine production, and activation of a variety of cells of the innate and adaptive immune systems. Most notably, repeated administration of alpha-GalCer to mice favors the generation of conventional T lymphocytes producing T helper (Th) type 2 cytokines such as IL-4 and IL-10. These findings suggest that alpha-GalCer can modulate inflammatory conditions that are mediated by pathogenic Th1 cells. Indeed, recent studies have demonstrated that alpha-GalCer prevents the development of Type 1 diabetes in non-obese diabetic mice and central nervous system inflammation in mouse models of multiple sclerosis. Collectively, these studies provide a solid foundation for the development of NKT cell ligands as pharmacological agents for treatment of autoimmune diseases.