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Multiple mechanisms are involved in regulation of host response to microbiota to maintain the intestinal homeostasis. Th17 cells are enriched in the intestinal lamina propria under steady conditions. Many studies have demonstrated that microbiota-reactive Th17 cells in the intestines mediate the pathogenesis of inflammatory bowel diseases. However, clinical trials of anti–interleukin-17A or anti–interleukin-17RA antibodies in patients with Crohn's Disease show no improvement or even exacerbation of disease. Accumulating data has also indicated that Th17 cells may provide a protective effect as well to the intestines from inflammatory insults under homeostasis regulation, even under inflammatory conditions. Thus both proinflammatory and anti-inflammatory functions of intestinal Th17 cells have emerged under various conditions. In this review article, we will summarize recent progresses of Th17 cells in regulation of intestinal homeostasis and in the pathogenesis of inflammatory bowel diseases.