The pathophysiology of irritable bowel syndrome (IBS) is still unknown. However, several lines of epidemiological, physiological, and clinical data suggest a role for intestinal bacteria in the pathogenesis of the disorder. Recent microbiology studies demonstrated differences in the composition of the intestinal microbiota between patients with IBS and healthy individuals. In addition, physiological studies have shown that manipulation of the intestinal microbiota by antibiotics, prebiotics, or probiotics can affect intestinal functions (eg, motility and sensation) relevant in the pathogenesis of IBS. Several randomized control trials comparing the effects of probiotics versus placebo in IBS have been published. Despite considerable differences in study design, dosing regimens, probiotic species used, and reported clinical end points, the current data indicate improving IBS symptoms and reducing the risk of persistent IBS symptoms. The data on the use of probiotics in children with IBS is more limited but is also suggestive for beneficial effects. The inconsistencies between the studies underline the need to look at each probiotic product separately for specific conditions, symptoms, and patient populations. This review article discusses the rationale for targeting the intestinal microbiota in patient with IBS and provides an overview and a critical evaluation of the currently available clinical data on the use of probiotics in the treatment of patients with IBS.