The T help cell 1(Th1) and Th2 cell classification have provided the framework for understanding CD4⁺ T cell biology and the interplay between innate and adaptive immunity for almost two decades. Recent studies have defined a previously unknown subset of the CD4⁺ T cell effectors, the Th17 lineage. The uncover on the differentiation of IL-17-producing effector T cells from naive T cell precursors provides insights into mechanisms by which signals from cells of the innate immune system guide alternative pathways of Th1, Th2 or Th17 development. Th17 lineage has an important role in autoimmune and inflammatory diseases. This promises to change our understanding on the immune regulation, immune pathogenesis and host defense. The identification, differentiation and immune function of the new subsets of Th cells, Th17 cells will be reviewed.