Interleukin 7 (IL)-7 is a pleiotropic, non-redundant cytokine necessary for the development of B and T lymphocytes, in particular gamma delta T cell receptor-positive cell differentiation. The cytokine can function as a cofactor during myelopoiesis and the generation of cytotoxic T cells and natural killer cells, can activate monocytes/macrophages, and support the survival of mature T cells. A role for IL-7 in promoting the formation of Peyer's patch anlage has also been demonstrated. IL-7 is constitutively expressed in the thymus, bone marrow stromal cells, epithelial and dendritic cells, keratinocytes, as well as in fetal and adult liver. IL-7 acts on various cells through its receptor (IL-7R), a heterodimer consisting of an alpha chain (CD127) that specifically binds IL-7 and a common c chain (CD132) shared by other cytokine receptors. The receptor is expressed on bone marrow progenitor cells, lymphoid T and B precursors, and mature T cells. IL-7 activity towards murine endothelial cells has been recently described. The presence of IL-7R on human endothelial cells has also been demonstrated. Several therapeutic applications of recombinant IL-7 have been proposed. These have focused on the enhancement of lymphopoiesis, promotion of stem cell engraftment, and the anti-tumor activity of the cytokine.