Natural killer T cells (NKT cells) play a central role in regulating immune responses influencing conditions ranging from autoimmune to infectious diseases. NKT cell responses are induced by recognition of glycolipid antigens presented by CD1d, an antigen presentation protein. In the last 10 years great strides have been made in understanding the types of glycolipids recognized by NKT cells. These advances have included determination of the lipid and carbohydrate recognition requirements for stimulation and identification of "natural" antigens for these cells.