Human gammadeltaT cells recognize pyrophosphomonoesters such as isopentenyl diphosphate and (E)-4-hydroxy-3-methylbut-2 enyl diphosphate derived from microbial pathogens. In addition, they display cytotoxic activity against various tumor cells in an NK receptor-dependent manner. When tumor cells are treated with nitrogen-containing bisphosphonates, gammadeltaT cells exhibit potent anti-tumor activity in a gammadeltaTCR-dependent manner. Based on these findings, gammadeltaT cells have attracted considerable attention in tumor immunotherapy. When gammadeltaT cell frequencies are relatively low, however, it is practically difficult to expand gammadeltaT cells. IL-18 has been recently shown to facilitate the development and proliferation of helper NK cells, which in turn promoted the expansion of gammadeltaT cells. The finding may contribute to the development of novel immunotherapeutic strategies for various cancers.