Th17 cells are a newly identified population of CD4 Th cells characterized by interleukin 17 production and are therefore named “Th17”. Interleukin 17 (IL-17; IL-17A) is one of the key cytokines for the activation, recruitment, and migration of neutrophils, so Th17 cells have been suggested to play an important role in responses against extracellular bacteria and fungi, in which granulocyte infiltration is highly protective. Among the other cytokines secreted by Th17 cells are IL-22, IL-26, IL-6, and TNF-α. Th17 cells are considered to be involved in the pathogenesis of various inflammatory, autoimmune, and allergic diseases. Th17 cells were found in both mouse and human tumors; however, their role in the tumor microenvironment is still poorly understood IL-17, as a pleiotropic cytokine, may suppress or promote tumor growth since it was shown that it stimulated tumor growth by inducing tumor vascularization or enhancing inflammation, but some other studies revealed also opposite roles for Th17 cells in human tumors. It seems that Th17 cells may play distinct roles in cancer depending of tumor immunogenicity, the stage of development, and the impact of inflammation and angiogenesis on tumor pathogenesis.