CD4 T cells are key components of the immune system that shape the anticancer immune response in animal models and in humans. The biology of CD4 T cells is complex because naïve T cells can differentiate into various subpopulations with various functions. Recently, a new population called Th9 cells was described. These cells are characterized by their ability to produce IL9 and IL21. They were first described in the context of parasite infections and allergic processes. However, some reports described their presence in the tumor bed in mice and humans. Their high secretion of IL9 and IL21 in the tumor bed contributes to their anticancer functions. Indeed, these cytokines trigger the activation of dendritic cells, mast cells, natural killer cells, and CD8 T cells to mount an antitumor immune response, thus explaining the remarkable ability of Th9 cells to control tumor growth. This review summarizes the latest advances in the Th9 field in cancer and focuses on their potential role as new tool for cell therapy. Cancer Res; 75(3); 475–9. ©2014 AACR.