Enteropathogenic Escherichia coli (EPEC) is associated with infantile diarrhoea and remains a major cause of paediatric morbidity and mortality in developing countries. Enterohaemorrhagic Escherichia coli (EHEC) is distinguished from EPEC by secretion of a Shiga-like cytotoxin, which can cause bloody diarrhoea, haemorrhagic colitis and haemolytic uraemic syndrome. EPEC and EHEC produce characteristic attaching and effacing (A/E) lesions on cultured cells and intestinal mucosae in vitro, and in vivo A/E intestinal lesions have been shown for EPEC in man, but not EHEC. The first gene to be associated with A/E lesion formation, the eae gene, encodes an outer membrane protein called intimin, and different intimin types have been recognised. The site of intestinal adhesion has been linked to intimin type in a pig model. Using in vitro organ culture (IVOC), one EHEC O157:H7 strain has been shown to produce A/E lesions on human Peyer's patches of the terminal ileum, but it did not adhere to other intestinal regions. The work in this thesis describes the use of IVOC, electron microscopy and molecular biology a) to study the interaction of EPEC and EHEC with human intestinal mucosa, and b) to address the phenomenon of tissue tropism of EPEC and EHEC. IVOC of EPEC and EHEC serogroups expressing different intimin types, confirmed the A/E lesion capability of EHEC on human tissue, demonstrated the FAE restricted phenotype of O157:H7 and O103:H- EHEC, and showed a variable tropism in EPEC strains and O26:H11 EHEC. Intimin exchange studies confirmed its tissue tropism role. Site-directed mutagenesis of intimin established that other, unidentified, factors also contribute. Colonic A/E lesion formation was shown in a minority of IVOC's, leading to the study of factors that might promote large bowel colonisation. Passage, fimbrial expression, mannose, activation, extended IVOC, and infected HEp-2 cell co-culture did not influence tropism. The lpf-like operons of EHEC were chosen for analysis, as lpf in Salmonella typhimurium is reported to mediate adhesion to Peyer's patches. Deletion in lpf-like operons in two EHEC strains altered tissue tropism in IVOC, giving the first example of a gene outside the LEE which influences intestinal colonisation.