NKT cells utilize a restricted αβ TCR repertoire that recognizes glycolipids in association with CD1d. The recent development of fluorescent CD1d tetramers loaded with the synthetic glycolipid α-galactosyl-ceramide has led to a clearer definition of NKT-cell subsets as well as important insights into their developmental origin. As many as four subsets may exist, differing in NK1.1 expression, TCR repertoire and dependence on CD1d and various glycolipids for development. Two different lineage-commitment models have been proposed, with most evidence favoring a byproduct of conventional-T-cell development.