Both thymic and extrathymic T lineage development are characterized by cytokine-dependent regulation of complex proliferative, differentiative, and anti-apoptotic processes. The role of the gammac-dependent cytokines in this program has been interpreted as limited to the activity of IL-7. However, through the analysis of double knock-out mice, which lack signaling through the IL-7R and other gammac-dependent cytokines, we revealed a role for IL-15 in the production of early thymic pro-T cells. Although IL-2 does not function in the production of thymocytes, thymic restoration of IL-2R expression prevented fatal autoimmunity associated with IL-2- or IL-2R-deficient mice, suggesting that IL-2R functions non-redundantly at the level of the thymus to regulate self-reactivity. Moreover, IL-2, IL-7, and IL-15 also extend their developmental effects beyond the thymus to other sites of T lymphocyte production, including the gut. Here, their redundant and non-redundant activities are directly correlated to the development of phenotypically diverse subsets of intestinal intraepithelial lymphocytes.