Probiotics and gut microbiota have a significant impact on gut homeostasis in the host. Recent clinical studies demonstrated the ameliorative features of several kinds of probiotics in intestinal disorders, such as inflammatory bowel diseases (IBDs). Interleukin (IL)-17 is a potent inflammatory cytokine, and T-helper (Th)17 cells and other IL-17-producing cells are involved in the pathogenesis of IBD. Multiple mechanisms of action have been suggested to explain the protective anti-inflammatory effects of probiotics in intestinal inflammation, including the immunoregulation and suppression of Th17 activity and IL-17 production in part by signaling through pattern-recognition receptors such as Toll-like receptor family. However, steady-state Th17 cells have an important role in host defense against fungi and bacteria. Interestingly, recent studies revealed that specific commensal bacterial species such as segmented filamentous bacteria (SFB) induce the accumulation of Th17 cells in the small intestine in many species, including mice. It is important to determine the mechanisms by which intestinal Th17 cells are induced by SFB and whether these or other bacteria with similar properties are present in the human intestine. This brief review focuses on the interaction between probiotics/microbiota and Th17 cells during inflammation (war) and during steady-state homeostatic regulation (peace).