Psoriasis is an inflammatory disease with dynamic interactions between the immune system and the skin. Recent studies have demonstrated that the interleukin (IL)-23//T-helper (Th)17 cell axis plays an important role in the pathogenesis of psoriasis. Here, the biology and function of Th17 cells as well as the crucial role of IL-23 in the context of the Th17 cell-dependent chronic inflammation in psoriatic skins are reviewed. Recent study about the role of the IL-23//Th17 axis in the pathogenesis of psoriasis-like lesions in K5.Stat3C transgenic mice is also discussed. This model mouse for psoriasis not only verifies the therapeutic efficacies of biologics that specifically target the IL-23//Th17 axis, but also clarifies the pathogenesis of psoriasis.