Natural killer T (NKT) cells are innate-like lymphocytes with immunoregulatory properties. They express an invariant and conserved T-cell receptor that recognizes glycolipids presented by CD1d. Manipulation, activation or loss of NKT cells has profound effects on diverse immune functions and can influence desirable responses to infectious organisms and tumors, as well as undesirable responses to autoantigens. Here, we discuss recent progress in the identification of natural endogenous and exogenous CD1d-presented antigens, the understanding of factors that regulate development of NKT cells in the thymus and of factors that influence their function in the periphery. Clinical implications of these findings are briefly touched on.