Interleukin-7 (IL-7) was initially isolated more than 10 years ago. 1-4 Nevertheless, the complete set of physiologic roles for this cytokine, especially those involving lymphocyte homeostasis, have only recently been elucidated. After the initial descriptions of effects on B-cell precursors, recognition that IL-7 also has marked activity on immature5-7 and mature8 T cells soon followed. Information from gene-deleted mice showed IL-7 is a nonredundant cytokine for murine T and B lymphopoiesis. 9, 10 Mutations in the ɑ chain of the IL-7 receptor in patients with severe combined immunodeficiency (SCID) confirmed that IL-7 is indispensable for T-cell development in humans. However, the presence of B cells in these individuals suggests important differences between the role of IL-7 in murine and human lymphocyte development. 11 IL-7 also has potent effects on mature T cells. Recent work has shown that IL-7 is a critical modulator of low-affinity peptide-induced proliferation, which is a central feature of the homeostatic regulation of T-cell populations. 12, 13 Furthermore, circulating levels of IL-7 increase in response to T-cell depletion, suggesting a role in T-cell regeneration. 14-16 Importantly, the primary sources of IL-7 are non–marrow-derived stromal and epithelial cells. Thus, IL-7 is a pleiotropic cytokine with central roles in modulating T-and B-cell development and T-cell homeostasis. The potency and breadth of effects suggest that IL-7 administration or neutralization of IL-7 may allow the modulation of immune function in patients with lymphocyte depletion, vaccine administration, or autoimmunity.