Accordingly, the current study has three aims: 1) to evaluate the effectiveness of group cognitive behaviour therapy for IBS on IBS symptomatology and quality of life; 2) to examine whether there are any changes in the gut microbiome from pre to posttreatment; and 3) to examine whether changes in the gut microbiome following cognitive behaviour therapy mediate changes in IBS symptomatology and quality of life. The current study has the potential of increasing knowledge about the role of the gut microbiome in IBS as well as the mechanisms of cognitive behaviour therapy for IBS.