A unique lymphocyte population, CD1d‐restricted NKT cells, has been revealed to be a key player in both the innate and acquired immune responses, including antitumor effects. Recent studies revealed that at least two subsets of CD1d‐restricted NKT cells exist: type I, having invariant Vα14 receptor; and type II, having heterogeneous non‐Vα14 receptor. The specific glycolipid ligand, α‐GalCer, effectively stimulates mouse and human type I NKT cells. The activation of type I NKT cells substantially influences function of other various cell types, particularly DC, NK cells, CD4 Th1 cells, and CD8 cytotoxic T cells, all contributing to the antitumor immune responses. Recent studies also indicated that, unlike type I NKT cells, type II NKT cells have a potential to repress antitumor immune responses. In this review, we summarize the characteristics of the antitumor immune responses mediated by both mouse and human CD1d‐restricted NKT cells and discuss their potential in clinical applications against cancer. (Cancer Sci 2006; 97: 807–812)