Uropathogenic Escherichia coli (UPEC) are the most important group of microorganisms responsible for urinary tract infection. UPEC differ from non-pathogenic E. coli and from other E. coli pathotypes by the production of specific virulence factors, which enable the bacteria to adhere to uroepithelial cells and to establish urinary tract infections. Besides adherence factors, toxins, 'modulins', capsules, iron uptake systems and other bacterial products contribute to the virulence of the strains. The respective genes are frequently located on large pieces of DNA called 'pathogenicity islands' (PAIs). PAIs form (unstable) regions of the genome of UPECs, which are often associated with tRNA genes. Using various molecular techniques, the functions of PAI encoded gene products have been studied. The usage of DNA arrays give answers to questions on the distribution of PAIs among various enterobacteria and on the expression of the different genes under in vitro and in vivo conditions. In addition, assumptions can be made on the evolution of these important pathogens.