CD4+ T cells have long been grouped into distinct helper subsets on the basis of their cytokine-secretion profile. In recent years, several subsets of innate lymphoid cell have been described as key producers of these same Th-associated cytokines. However, the functional relationship between Th cells and innate lymphoid cells (ILCs) remains unclear. We show in this study that lineage-negative ST2+ ICOS+ CD45+ type 2 ILCs and CD4+ T cells can potently stimulate each other’s function via distinct mechanisms. CD4+ T cell provision of IL-2 stimulates type 2 cytokine production by type 2 ILCs. By contrast, type 2 ILCs modulate naive T cell activation in a cell contact–dependent manner, favoring Th2 while suppressing Th1 differentiation. Furthermore, a proportion of type 2 ILCs express MHC class II and can present peptide Ag in vitro. Importantly, cotransfer experiments show that type 2 ILCs also can boost CD4+ T cell responses to Ag in vivo.