T helper 17 (T_H17) cells have been extensively studied since their discovery 10 years ago, primarily because of their known pathogenic role in many inflammatory diseases. Substantial progress has been made in understanding their development, regulation and functional activities, and genome-wide transcriptomic analysis has identified regulatory networks, nodes and interactions that provide vital clues for further studies. In this Review, we describe recent studies that have revealed the dichotomous nature of T_H17 cells, which on the one hand allows these cells to be pathogenic drivers of inflammatory disorders and on the other hand allows them to support the integrity of the intestinal barrier in a non-inflammatory manner.