Most infections of the upper urinary tract respond promptly to antibiotic therapy and imaging is not necessary. Patients with urinary obstruction, diabetes, or immunocompromise are more likely to develop complicated infection, abscess, or have unusual organisms. Chronic granulomatous processes involving the kidney are usually related to recurrent bacterial infections. Again, stone disease or obstruction is often an underlying problem. In those patients who do not respond promptly to treatment or have a more complicated clinical picture, imaging can assess the severity and extent of disease. CT scan is the study of choice for diagnostic evaluation in these patients and directs percutaneous intervention when appropriate. Placement of drainage catheters is often curative but also may allow the patient to stabilize until surgical treatment is accomplished. One exception is the diagnosis of pyonephrosis, which may be accomplished more easily by ultrasound. In these cases, PCN placement is generally needed and is performed under fluoroscopic guidance. Ultimately, however, definitive surgical intervention often is needed to relieve the underlying obstruction.