Escherichia coli is the most common cause of complicated as well as uncomplicated urinary tract infections (UTIs). Most of these uropathogenic E. coli (UPEC) strains exhibit certain virulence factors (VFs), including adhesins, iron uptake systems, synthesis of cytotoxins, and specific O:K:H serotypes. Molecular epidemiological studies of UPEC have contributed to the discovery of uropathogenic VFs, to an understanding of the pathogenesis of UTIs as ascending infections, and to the clarification of genetic linkages between different virulence genes such as pathogenicity islands (PAIs), which are one of the mechanisms for horizontal VF gene transfers between the same or related species. Uropathogenic VFs not only play an important role individually but also work cooperatively in a fine-tuned manner with coordinated regulation and expression.