The irritable bowel syndrome is one of a group of functional gastrointestinal disorders within the Rome classification system that is characterized by abdominal discomfort or pain associated with a change in stool habit. It is a multidetermined biopsychosocial disorder in which physiological, psychological, behavioral, and environmental factors may contribute to the clinical expression of the disorder. These can include: (1) early life (e.g., genetic or environmental) factors; (2) physiological factors including increased motor reactivity, visceral hypersensitivity, which may be enabled by postinfectious events, and dysregulation of brain-gut communication (e.g., altered central pain control mechanisms). In addition, psychosocial factors including psychiatric co-morbidity, major trauma (e.g., abuse history), and maladaptive coping may amplify the clinical expression of the disorder and its outcome. Currently, clinical outcome has become understood in terms of global symptom relief and health-related quality of life.