A complicated urinary tract infection (UTI) has relapsing and refractory characteristics, and is sometimes life-threatening because of patient predisposing factors as well as the recent worldwide spread of multi-drug resistant bacteria. Patients with complicated UTI should be treated with effective antimicrobial therapy along with appropriate urological intervention to remove predisposing factors when the symptoms are associated. By contrast, routine use of antimicrobial prophylaxis for asymptomatic bacteriuria (ASB) is not recommended, as that would contribute to an increase in even more resistant pathogens. Here, four classifications of complicated UTI, which are considered to be clinically important for general urologists, are reviewed, including UTI in patients with diabetes mellitus (DM) and those with a neurogenic bladder, as well as catheter-associated UTI (CAUTI) and obstructive pyelonephritis secondary to urolithiasis. Appropriate treatment approaches can only be chosen by proper understanding of the etiologies of complicated UTI, as well as correct diagnostic strategies and treatment options.