As a group, functional gastrointestinal disorders are the most common gastrointestinal disorder seen by both generalists and specialists. These disorders can be frustrating to both patients and physicians as they are usually chronic in nature and difficult to treat. These disorders are associated with frequent healthcare visits, the scheduling of multiple, expensive diagnostic tests, and the use of both over-the-counter and prescription medications. All of these factors lead to a significant economic burden to society. In addition, functional gastrointestinal disorders are associated with a reduction in quality of life for the patient. Irritable bowel syndrome (IBS) is the most common of the functional gastrointestinal disorders. This syndrome has been the focus of a large number of research studies over the past two decades. These studies have resulted in a number of significant changes in our definition of IBS. In addition, these research studies have produced considerable changes in our understanding of the etiology and pathogenesis of IBS. In this section, we will review some of the evolutionary changes that have occurred in IBS. We will discuss how the definition of IBS has changed, consider our evolving strategies to evaluate and diagnose IBS, and finally, provide a brief overview of treatment options for this common disorder.