The term 'functional gastrointestinal disorder (FGID)' is used to define several variable combinations of chronic or recurrent gastrointestinal (GI) symptoms that do not have an identified underlying pathophysiology. In the absence of any objective marker, the identification and classification of FGIDs are based on symptoms. The most widely accepted classification is based on the 'Rome diagnostic criteria,' which have classified 24 FGIDs into oesophageal, gastroduodenal, bowel, biliary, anorectal and abdominal pain subcategories. Classification into mutually exclusive categories has been useful for performing epidemiological studies in homogeneous populations, but has inevitably lead to disregarding subjects with overlapping FGIDs, or with a not sufficiently standardised symptom presentation. The epidemiology of FGID is still in its infancy, as indicated by the lack of epidemiological data for many FGIDs and the widely different incidence and prevalence rates reported for the most frequently occurring and investigated FGIDs: irritable bowel syndrome (IBS), dyspepsia, constipation and oesophageal disorders. Epidemiological studies and the definitions of the various FGIDs need to be further improved and standardised.