Fimbriae are long, threadlike protein polymers found on the surface of many strains of Escherichia coli. The presence of fimbriae has been found to be significantly correlated with pathogenicity, and specific fimbriae confer on pathogenic strains the ability to adhere to and colonize various specific host epithelia. A surprising variety of fimbrial adhesions have been found in E. coli; several unrelated groups of such proteins have been extensively characterized. Fimbriae of E. coli are strongly immunogenic. Their continuous interaction with host defenses has imposed heavy selective pressure on them, leading to a fine balance between functional necessity and maximal immunologic diversity. These important virulence factors may be exploited in various ways in the production of vaccination agents. Vaccines based on E. coli fimbriae have proved extremely successful in the veterinary sector, and several types of immunization are currently being tested in humans.