Urinary tract infections are among the most frequent infections encountered in developed countries. The vast majority of community-acquired urinary tract infections are caused byEscherichia coli. However, other bacterial species play an important role in nosocomial urinary tract infections. All these species are equipped with a variety of virulence factors. The best characterized are those fromEscherichia coli. Among the first virulence factors that come into play during establishment of a urinary tract infection are adhesins. Besides their primary function as adhesin molecules several other additional functions can now be attributed to these organelles. Adhesins may also function as invasins, promote biofilm formation and transmit signals to epithelial cells resulting in inflammation. Furthermore, subunit proteins of adhesins seem to be promising vaccines. Later in infection, toxins seem to enhance virulence. However, for cytotoxic necrotizing factor type 1 this is controversial. Many virulence factors of uropathogenic bacteria are encoded by foreign DNA stretches inserted into the core genome. These pathogenicity islands or islets were obviously acquired via horizontal gene transfer creating new pathotypes more efficient in establishing infection. The role of new virulence factors and the new functions of already known virulence factors will be discussed as well as the concept of the composite genome of uropathogenicEscherichia coli.