The functionally different CD4 T helper cell subsets known as T helper 1 (Th1) and T helper 2 (Th2) display a unique and different cytokine profile. Abnormal skewing toward Th1 or Th2 cells has been suggested to play an important role in the pathogenesis of autoimmune disorders and in inflammatory and allergic diseases. The Th1/Th2 paradigm continues to serve as a model to understand the pathogenesis of several pathologic conditions and provides the rationale for the development of new strategies for treating and preventing these diseases. Over the past year, efforts have continued to increase our understanding of the mechanisms underlying the development and regulation of Th cells and their pathogenic role and therapeutic potential in the induction and treatment of clinic diseases. Several teams of researchers have further examined the association of Th1/Th2 inducing factors and allergic disease and intestinal inflammatory diseases. The protective effect of helminth infection on allergy and the role of regulatory T cells in both Th1- and Th2-mediated diseases have been further examined.