Functional gastrointestinal disorders (FGIDs) are common in the community, are of chronic nature and pose a significant health care burden. The causes and pathogenetic mechanisms of FGIDs are not fully known. FGID are classified into three major domains: functional dyspepsia, functional bowel disorders including irritable bowel syndrome (IBS), and heartburn. The aim was to study the natural history of FGID in the Icelandic population over a 10-year period. Only one other study has been performed on FGIDs using the same methodology, in Olmsted County Minnesota, USA and thus providing the possibility of a direct comparison. Secondary objectives of the study were: 1) to compare the prevalence and stability of IBS according to the Manning criteria, Rome II, Rome III subtypes, and self-reported IBS over a 10 year period: 2) to study IBS and dysmenorrhea in women and to assess the change in IBS over menopause: 3) to study if and how physicians use the IBS criteria and to assess treatment strategies among physicians for IBS patients. This book should help shed some light on this health care problem, and should be especially useful to professionals in gastroenterology.