Since the mid-1940s, Escherichia coli has been recognized as a cause of diarrhoea. Subsequently it has been shown that at least five different pathogenic mechanisms are used to cause disease. Enterotoxigenic E. coli (ETEC) and enteropathogenic E. coli (EPEC) produce a noninflammatory diarrhoea, whereas enteroinvasive E. coli (EIEC), enterohaemorrhagic E. coli (EHEC) and enteroaggregative E. coli (EAggEC) produce an inflammatory diarrhoea. ETEC are a major cause of diarrhoea in infants (up to three episodes per year) and travellers. They produce diarrhoea by attaching to the small intestinal mucosa and elaborate one or both of heat labile and heat stable toxins. EPEC attach firmly to the intestinal mucosa leading to dissolution of the brush border by inducing vesiculation of the microvilli. This process is known as attaching-effacement, and in the jejunum and ileum results in a loss of brush border disaccharidase enzymes and a large area of absorptive surface. EPEC are a major cause of summer diarrhoea in infants and neonatal diarrhoea. EIEC attach to colonic enterocytes, penetrate by an endocytotic mechanism and replicate therein. This results in necrosis and stripping of large areas of colonic mucosa and a dysentery similar to but usually less severe than Shigella dysentery. EHEC produce attaching-effacement to the terminal ileal and colonic mucosa and release the toxins, verocytotoxin (VT) 1 or 2. These kill colonic enterocytes and produce haemorrhagic colitis. In addition, VT can damage vascular endothelial cells, leading to haemolytic uraemic syndrome. The role of EHEC in diarrhoea in children in the tropics is not known. The most recently described group are the EAggEC. They damage and blunt colonic villi by haemorrhagic necrosis, although the precise pathogenic mechanisms are unclear. EAggEC are a major cause of chronic diarrhoea in children. Although certain O-serotypes are associated with the different enteropathic E. coli, serotyping is not sufficiently specific or sensitive for use as a diagnostic tool. Specific diagnosis is expensive and time consuming and depends upon demonstration of the pathogenicity trait, and the pathogenicity gene(s) or their gene product(s). At present, to undertake such testing is not recommended for routine diagnosis but is most useful when surveys of the aetiology of acute and chronic diarrhoea are undertaken.