T helper (Th) 17 cells have recently been described as a third subset of T helper cells, and have provided new insights into the mechanisms that are important in the development of autoimmune diseases and the immune responses that are essential for effective antimicrobial host defense. Both protective and harmful effects of Th17 responses during infection have been described. In general, Th17 responses are critical for mucosal and epithelial host defense against extracellular bacteria and fungi. However, recent studies have reported that Th17 responses can also contribute to viral persistence and chronic inflammation associated with parasitic infection. It has become evident that the type of microorganisms and the setting in which they trigger the Th17 response determines the outcome of the delicate balancethat exists between Th17 induced protection and immunopathogenesis.