Small intestinal bacterial overgrowth (SIBO) syndrome is characterized by an imbalance of a complex microbiome in the gut. The underlying factors include anatomic abnormalities, (small intestinal obstruction, diverticular disease, fistulas, blind or afferent loops, and previous ileo-cecal resection), motility disorders (scleroderma, diabetic autonomic neuropathy, and ileus), and some conditions associated with failure of antibacterial defense mechanisms (achlorhydria, pancreatic exocrine insufficiency, and an immunosuppressed state). In recent studies, there is abundant evidence suggesting the relationship between SIBO and irritable bowel syndrome; however, it is not clear which disease is a primary factor to the other. Symptoms related to SIBO include diarrhea, bloating, malabsorption, weight loss, and anemia. The gold standard test for the diagnosis of SIBO is aspiration of jejunal fluid for culture. However, non-invasive hydrogen and methane breath tests are more widely used for the diagnosis, although they have several potential problems affecting the diagnostic accuracy. The treatment should be individualized, and usually consists of correction of the underlying predisposing disease, nutritional support, and cyclic or repeated courses of antimicrobials.