The irritable bowel syndrome (IBS) is the prototypic functional bowel disorder in terms of its heterogeneous nature, multifactorial pathogenesis and requirement for individualized diagnosis and treatment. The surge of scientific interest in IBS during the past two decades has brought increased awareness that both the traditional biomedical model of disease and a purely psychiatric explanation are inadequate keys to understanding and treating IBS. Most researchers and clinicians accept the ability of the multinational Rome symptom criteria to identify IBS in epidemiological surveys and play a dominant diagnostic role in clinical practice. Many patients with symptoms typical of IBS can be confidently diagnosed after the performance of flexible sigmoidoscopy and a complete blood count. IBS is a common functional disorder which is best understood from the biopsychosocial concept of illness in which both biological and psychosocial factors influence symptom experience, behavior and outcome.