Natural killer T (NKT) cells are αβ T cells that express a semi-invariant T-cell receptor (TCR) along with natural killer (NK) cell markers and have an innate cell-like ability to produce a myriad of cytokines very quickly upon antigen exposure and subsequent activation. These cells are diverted from conventional single positive (SP) T-cell fate at the double positive (DP) stage, where TCR-mediated recognition of a lipid antigen presented on a CD1d molecule promotes their selection into the NKT lineage. Although many key regulatory molecules have been shown to play important roles in the development of NKT cells, the mechanism of lineage specification and acquisition of effector functions in these cells still remain to be fully addressed. In this review, we specifically discuss the role of a family of class-I helix-loop-helix proteins known as E proteins, and their antagonists Id proteins in NKT celldevelopment. Recent work has shown that these proteins play key roles in invariant NKT (iNKT) development, from the invariant TCR rearrangement to terminal differentiation and maturation. Elucidating these roles provides an opportunity to uncover the transcriptional network that separates NKT cells from concurrently developed conventional αβ T cells.