CD1d-restricted natural killer T (NKT) cells are innate lymphocytes that play a regulatory role during an immune response. The identification of alpha-galactosylceramide (alpha-GalCer), a marine sponge-derived glycosphingolipid, as a potent stimulator of NKT cells led many laboratories to investigate the effects of NKT cell activation on the regulation of immune responses. These studies revealed that alpha-GalCer induces rapid and robust cytokine production by NKT cells, secondary activation of a variety of innate and adaptive immune cells, and modulation of Th cell responses. Further, alpha-GalCer influences disease progression in a variety of experimental models of autoimmunity and inflammation in mice, including models for type 1 diabetes, multiple sclerosis, rheumatoid arthritis, systemic lupus erythematosus, inflammatory bowel disease, and atherosclerosis. While these studies have raised significant enthusiasm for manipulation of NKT cells as a means of preventing autoimmunity in the clinical setting, there are significant concerns regarding the safety of repeated alpha-GalCer injections in human subjects.