IL-17-producing T cells have recently been classified as a new effector T-cell subset, termed Th17, which is distinct from Th1, Th2 and Treg subsets. There has been much progress in the past year, leading to identification of the molecular mechanisms that drive differentiation of Th17 T cells. This has helped to clarify many aspects of their role in host defense as well as in autoimmunity. Nevertheless, many intriguing questions remain to be answered regarding the regulation of Th17-mediated responses as well as their interactions with the other T-cell subsets. Furthermore, the role of pathogens and pathogen-derived molecules in influencing effector T-cell polarization needs to be re-evaluated in the light of the differentiation conditions that favor Th17 T-cell responses.