Abstract: Irritable bowel syndrome (IBS) is a common functional gastrointestinal disorder with an estimated worldwide prevalence of 10–20%. IBS can be associated with severe gastrointestinal symptoms, including abdominal pain, bloating, and altered bowel function. Although the causes of IBS remain undefined, recent research has increasingly suggested roles for gut flora in IBS. These roles involve postinfectious IBS, which can occur after a single episode of acute gastroenteritis, and small intestinal bacterial overgrowth, in which elevated populations of aerobic and anaerobic bacteria cause abdominal pain and altered bowel function. More recently, potential roles for methanogens in contributing to IBS subtypes have also been identified. In this paper, we review the different mechanisms by which gut flora may contribute to IBS and also discuss the efficacy and safety of various antibiotic therapies for treating IBS symptoms.