Urinary tract infection (UTI) is one of the most common bacterial infections, and the incidence in women is much higher than in men. The diagnosis of a UTI can be made based on a combination of symptoms and a positive urine analysis or culture. Most UTIs are uncomplicated UTIs, defined as cystitis in a woman who is not pregnant, is not immunocompromised, has no anatomical and functional abnormalities of the urogenital tract, and does not exhibit signs of tissue invasion and systemic infection. All UTIs that are not uncomplicated are considered to be complicated UTIs. Differentiation between uncomplicated and complicated UTIs has implications for therapy because the risks of complications or treatment failure are increased for patients with a complicated UTI. Asymptomatic bacteriuria (ASB) is defined as the presence of a positive urine culture collected from a patient without symptoms of a UTI. Concerning the complicated UTI, it is possible to make a differentiation between UTI with systemic symptoms (febrile UTI) and UTI in a host, which carries an increased risk to develop complications of this UTI. Febrile UTIs are urosepsis, pyelonephritis, and prostatitis. A complicated host is defined as one that has an increased risk for complications, to which the following groups belong: men, pregnant women, immunocompromised patients, or those who have an anatomical or functional abnormality of the urogenital tract (e.g., spinal cord-injury patients, renal stones, urinary catheter).