Interleukin-7 (IL-7) is a cytokine produced predominantly by stromal cells of the thymus and bone marrow and is essential for lymphopoiesis. This paper reviews the importance of IL-7 and its receptor (IL-7R) in T-cell genesis, peripheral survival, expansion and memory T-cell development. IL-7 is of particular importance in lymphopenic conditions. Its expression is up-regulated in a number of lymphopenic conditions including: marrow ablation prior to bone marrow transplantation, marrow suppression following chemotherapy and human immuno-deficiency virus (HIV) infection. Plasma IL-7 levels inversely correlate with CD4+ T-cell counts in these conditions. Animal models suggest that IL-7 improves immune reconstitution through increasing thymic output and, perhaps more importantly, through antigen-independent homeostatic driven proliferation in the periphery. Given the promising preliminary data on the use of IL-7 adjuvant therapy in simian immuno-deficiency virus (SIV) infected non-human primates, IL-7 has recently moved into Phase I/II clinical trials of its role as a possible adjuvant therapy for cancer and HIV infection. This paper discusses important considerations such as the possible negative impacts of IL-7 on increased viral infectivity, the induction of autoimmunity and risk of neoplastic events. Successful use of IL-7 will rely on further understanding of the regulation of the component parts of the IL-7R system. Ultimately this understanding may lead to therapeutics that manipulate and optimise signalling through the IL-7/IL-7R system.