The term urinary infection defines those processes located in the urinary tract due to the presence of microorganisms in the urine, particularly bacteria. Regardless the sex or age of the patient, they represent the second most frequent type of infection, after respiratory tract infections, not only in outpatients but also in hospitalized patients. There is an ongoing interest about the etiology and pathogenesis of urinary infections in order to identify the virulence-factor profiles of bacterial isolates and the characteristics of the infection-prone individuals. Urinary infections are caused by a wide range of pathogens, specially gramnegative rods (E. coli, P. mirabilis, Klebsiella). However, there is an increasing evidence of the role as pathogens of some microorganisms that have long been regarded as apathogenic like E. faecalis and the Staphylococcus group. The treatment of such infections represents a challenge for the future mainly in the fields of control of antimicrobial resistance and vaccine development.