Interleukin-31 (IL-31) is a recently discovered T-cell cytokine closely related to IL-6 type cytokines and is preferentially produced by T helper type 2 cells. IL-31 activity is mediated through the ligand-induced oligomerization of a dimeric receptor complex containing IL-31 receptor A and oncostatin M receptor. In response to IL-31 binding, these proteins activate the JAK/STAT and the AKT signaling pathways. RNA levels of IL-31 receptor A and oncostatin M receptor are induced in activated monocytes but are expressed constitutively in epithelial cells. IL-31, when overexpressed in transgenic mice, results in the development of pruritis, alopecia, and skin lesions and in humans may result in atopic dermatitis, suggesting that IL-31 may represent a novel target for antipruritic drug development.