Urinary tract infections (UTIs) include cystitis (infection of the bladder/lower urinary tract) and pyelonephritis (infection of the kidney/upper urinary tract). The pathogenesis of UTI begins with colonization of the vaginal introitus or urethral meatus by uropathogens from the fecal flora, followed by ascension via the urethra into the bladder. Pyelonephritis develops when pathogens ascend to the kidneys via the ureters. Pyelonephritis can also be caused by seeding of the kidneys from bacteremia. Simple UTIs occur in healthy women with normal anatomy, characterized by symptoms of cystitis in the absence of fever, flank pain and other signs of systemic illness. In contrast, acute complicated UTI is defined as UTI that has possibly extended beyond the bladder (UTI with fever or other systemic symptoms). Complicated UTIs are associated with: involvement of the upper urinary tract, anatomic abnormalities, urolithiasis, the presence of catheters, malignancy and immunosuppression. Escherichia coli is the most frequent cause of acute complicated urinary tract infections. Diagnostic imaging for UTI in adults is usually not necessary, unless there is concern about an alternative diagnosis or a complication. Computed tomography (CT) is frequently used for evaluation of patients with a complicated UTI. Ultrasound is another diagnostic option, and it is ideal because it does not involve radiation. The cornerstone of treatment of any bacterial infection, including a UTI, is antimicrobial therapy. General treatment strategies to eliminate an infection are the judicious use of antimicrobials, ensuring adequate hydration, and relieving any urinary obstruction or foreign bodies.