NK T cells are a lymphocyte lineage that is selected by CD1d and is characterized by the ability to rapidly secrete large amounts of both IFN-γ and IL-4 after TCR stimulation. Using reactivity to CD1d tetramers to define presumptive NK T cells, several NK T cell progenitor populations were characterized based upon NK marker expression and CD4 vs CD8 expression. The earliest populations were found to be negative for NK markers and could proliferate to IL-7, while mature NK T cells did not. The NK1. 1− NK T cell progenitors were capable of up-regulating NK1. 1 when transferred in vivo. Upon stimulation, the NK1. 1− populations secrete IL-4, but little IFN-γ. As the cells mature and up-regulate NK1. 1, they acquire the ability to secrete IFN-γ. Finally, the Tec family tyrosine kinase Itk is necessary for optimal NK1. 1 up-regulation and hence final maturation of NK T cells. The itk−/− mice also display a progressive decrease in NK T cells in older animals, suggesting a further role in peripheral maintenance.