Irritable bowel syndrome (IBS) is the most common disorder seen in gastroenterology practice. It is also a large component of primary care practices. Although the classic IBS symptoms of lower abdominal pain, bloating, and alteration of bowel habits is easily recognizable to most physicians, diagnosing IBS remains a challenge. This is in part caused by the absence of anatomic or physiologic markers. For this reason, the diagnosis of IBS currently needs to be made on clinical grounds. A number of symptom-based diagnostic criteria have been proposed over the last 15 years. The most recent of these, the Rome II criteria, seem to show reasonable sensitivity and specificity in diagnosing IBS. However, the role of the Rome II criteria in clinical practice remains ill defined. A review of the literature shows that, in patients with no alarm symptoms, the Rome criteria have a positive predictive value of approximately 98%, and that additional diagnostic tests have a yield of 2% or less. Diagnostic evaluation should also include a psychosocial assessment specifically addressing any history of sexual or physical abuse because these issues significantly influence management strategies and treatment success.