Symptoms consistent with the irritable bowel syndrome are remarkably frequent around the world. Irritable bowel syndrome prevalence ranges from 2.1% to 22%, depending on criteria used. Women are more frequently affected than men, but the reasons remain obscure; irritable bowel syndrome occurs in all age groups but there appears to be a modest decline in prevalence with advancing age again for unknown reasons. The incidence of irritable bowel syndrome per year has been estimated at approximately 1.5% in community subjects; annually only 0.2% of population will be diagnosed with irritable bowel syndrome. The natural history of irritable bowel syndrome is characterized by symptomatic flare ups and by a high rate of transition to other functional gastrointestinal diseases over the long term. Well recognized risk factors for irritable bowel syndrome include psychological distress and gastroenteritis. However, the association of psychological distress in some cases may reflect confounding factors and might be explained at least in part by cytokine production. Familial aggregation of irritable bowel syndrome occurs, and while the environment is key, twin studies generally support a genetic component in irritable bowel syndrome explaining up to 20% of the variability. Prior surgery may increase risk of irritable bowel syndrome. Early childhood trauma may be important; a low birth weight, nasogastric suction at birth, childhood abuse, and low socioeconomic status may carry an increased risk of suffering with irritable bowel syndrome as an adult. The role of diet remains uncertain but under-studied.