CD4+ T helper cells are critical mediators of the cellular immune response. For many years, it was thought that CD4+ T helper cells existed as a dichotomy of lineages identified as Th1 and Th2 based on their expression pattern of effector cytokines. This paradigm was challenged with the demonstration that IL-17 (also known as IL-17A) was produced and secreted as a homodimer or heterodimer (with IL-17F) by a unique subset of T helper cells with the subsequent identification of ROR gamma (t) as the transcription factor responsible for specifying the lineage differentiation of Th17 cells. Th17 cell development is also driven and regulated by the expression of transcription factors STAT3, AHR, BATF, IRF4, c-Maf, IKB zeta and ROR alpha. Th17 cells play a role in host defense against extracellular pathogens and there is significant evidence that aberrant regulation of Th17 cells plays a role in the pathogenesis of multiple inflammatory and autoimmune disorders.