Although there is a wide variability in symptoms, disorders of colonic motility are the most prominent features in irritable bowel syndrome (IBS). Stool weight is within the normal range but many patients appear to have abnormal rectal sensations. Straining even with soft stool is common. Dietary fibre stimulates ileocolonic flow and may induce more symptoms in IBS than normal. There is evidence of increased responsiveness of the IBS colon, both to the effect of eating and to stress. Defaecatory disorders are common and may reflect both increased or decreased rectal sensitivity. The normal colon is quiescent during sleep, but in IBS coma sleep is often abnormal, with more periods of arousal and the colon consequently more active. There is evidence of increased responsiveness to corticotrophin releasing factor, which mediates much of the effect of stress on the gut. Many patients show a sympathetic/vagal imbalance with relative excess of sympathetic influence in keeping with increased levels of psychological stress and anxiety. There is undoubtedly more than one cause of IBS and around 25% appear to develop symptoms after an infectious enteritis. This has effects on the entero-endocrine system which may take many years to subside.