The severity of an infection is a function of the resistance of the host and the virulence of the infecting strain. Infections of the urinary tract can be caused by a wide range of gram-negative and gram-positive bacteria, which also are constituents of the normal flora. Indeed, most uropathogens originate from the intestinal tract. Still, the virulence concept implies that bacteria associated with urinary tract infections differ from members of the indigenous flora not causing infections. This review attempts to summarize the mechanisms known to contribute to bacterial virulence in the urinary tract and their relevance in different patient groups.