ABSTRACTIdentification of CD4+Foxp3+ Tregs and Th17 modified the historical Th1–Th2 paradigm. Currently, the Th17–Tregs dichotomy provides a dominant conceptual framework for the comprehension of immunity/inflammation and tolerance/immunosuppression in an increasing number of diseases. Targeting proinflammatory Th17 cells or immunosuppressive Tregs has been widely considered as a promising therapeutic strategy in the treatment of major human diseases, including autoimmunity and cancer. The efficacy and safety of such therapy rely on a thorough understanding of immunobiology and interaction of these two subsets of Th cells. In this article, we review recent progress concerning complicated interplay of Th17 cells and Tregs. There is compelling evidence that Tregs potently inhibit Th1 and Th2 responses; however, the inhibitory effect of Tregs on Th17 responses is a controversial subject. There is increasing evidence showing that Tregs actually promote the differentiation of Th17 cells in vitro and in vivo and consequently, enhanced the functional consequences of Th17 cells, including the protective effect in host defense, as well as detrimental effect in inflammation and in the support of tumor growth. On the other hand, Th17 cells were also the most potent Th subset in the stimulation and support of expansion and phenotypic stability of Tregs in vivo. These results indicate that these two subsets of Th cells reciprocally stimulate each other. This bidirectional crosstalk is largely dependent on the TNF–TNFR2 pathway. These mutual stimulatory effects should be considered in devising future Th17 cell- and Treg-targeting therapy.