Irritable bowel syndrome (IBS) is one of the most common gastrointestinal disorders and it is characterized by episodes of abdominal pain and altered bowel functions. The specific bowel disturbances of diarrhea, constipation or an alternation between the two defines the IBS subtypes of diarrhea-predominant, constipation-predominant, and mixed or alternating IBS. Because of the abnormalities in bowel states associated with each IBS subtype, it is not likely that one agent would successfully treat all three subtypes. As a result, clinical trials have focused, for the most part, on one IBS subtype. Over the past 2 decades very few agents have achieved regulatory approval for the treatment of IBS. In the present article we review publications reporting on phase 2 and phase 3 studies evaluating agents to potentially be used in the treatment of patients with IBS.