Irritable bowel syndrome (IBS) is a common disorder with major health status and economic effects. Symptom criteria are of paramount importance in diagnosis, but differences among the Manning, Rome I, and Rome II criteria may lead to variable identification of people with the disorder. Practice guidelines are based on evidence and, to a greater degree, on consensus; therefore, experts vary on the specifics of ordering particular diagnostic tests. There is an overlap of IBS symptoms with those of celiac sprue, and selected patients should be tested for the latter disease. Symptom confusion with biliary pain and overlap with chronic pelvic pain could contribute to the predisposition of IBS patients to undergo cholecystectomy and hysterectomy. Development and documentation of effective therapy has been difficult, but depending on the selection of subgroups, there is evidence for usefulness of smooth muscle relaxants, loperamide, and antidepressants. Various forms of psychological therapy and new serotonin-modulating agents seem especially promising. The placebo effect of the physician-patient relationship has important therapeutic benefit.