The discovery of two distinct subsets of helper T cells, IFN-gamma-producing Th1 cells and IL-4-producing Th2 cells, about three decades ago enabled us to understand the immunopathology of cell-mediated and allergic inflammatory diseases in humans. The observation that T cell-mediated experimental autoimmune diseases can be induced in mice lacking Th1 and Th2 cell responses prompted many immunologists to hypothesize that there might be additional subsets in helper T cell population which mediate autoimmunity in the absence of Th1 and Th2 cells. Consequently, multiple independent research groups identified IL-17-expressing ROR gamma t(+)CD4(+) T cell population as a distinct subset of helper T cells which promotes autoimmune tissue inflammation. Subsequent studies have revealed that innate immune cells, including gamma delta T cells, NKT cells and innate lymphoid cells, also produce type 17 cytokines and contribute to tissue inflammation. In this review, we discuss our current understanding on the biology of IL-17 and the therapeutic potential of targeting IL-17 for the treatment of immune disorders in humans.