Naive CD4⁺ T cells undergo massive proliferation and differentiation into at least four distinct helper T cell subsets after recognition of foreign antigen–derived peptides presented by dendritic cells. Each helper T cell subset expresses a distinct set of genes that encode unique transcription factor(s), as well as hallmark cytokines. The cytokine environment created by activated CD4⁺ T cells, dendritic cells and/or other cell types during the course of differentiation is a major determinant for the helper T cell fate. This Review focuses on the role of cytokines of the common γ-chain (γ_c) family in the determination of the effector helper T cell phenotype that naive CD4⁺ T cells adopt after being activated and in the function of these helper T cells.