Five cytokines, IL-2, IL-4, IL-7, IL-9, and IL-15, form one group that is characterized by utilizing the common gamma chain (gamma c) as a receptor subunit. Examination of the phenotype of various cytokine or cytokine receptor "knockout" mice demonstrates that these cytokines are critical for normal lymphocyte development and subsequent functional activity of the peripheral immune compartment. This review summarizes the structural and functional properties of each of these five cytokines and their receptors, including the known redundant pathways for each cytokine or receptor. The contribution of these cytokines and receptors will then be considered in detail with respect to regulation of T lymphocyte development and homeostasis of the peripheral T cell compartment.