Interleukin-21 (IL-21) is a newly described cytokine, produced by activated CD4+ T cells. Since the discovery in 2000, IL-21 has been the object of intensive research because of its homology to IL-2, IL-4 and IL-15, and its ability to modulate both innate and adaptive immune responses. IL-21 mediates its functions through a heterodimeric receptor, composed of a specific subunit, termed IL-21 receptor (IL-21R) and the common gamma-chain, that is shared with IL-2, IL-4, IL-7, IL-9, IL-13, and IL-15 receptors. IL-21R is originally described on T, B and NK cells, which is in accordance with the cell types that mostly respond to IL-21. Indeed, IL-21 augments the proliferation of CD4+ and CD8+ T lymphocytes and regulates the profile of cytokines secreted by these cells, drives the differentiation of B cells into memory cells and terminally differentiated plasma cells, and moreover, enhances the activity of natural killer cells. More recently, IL-21R has also been documented on non-immune cells, raising the possibility that IL-21 is an important mediator in the cross-talk between immune and non-immune cells. As discussed in this review, the potential role of IL-21 in immune-mediated and allergic diseases would seem to suggest that either disrupting or enhancing IL-21 signaling may be useful in specific clinical settings.