The principal indication for renal ultrasound is in the diagnosis of ureteral calculi, which, if they cause obstruction, will give rise to unilateral hydronephrosis. Less commonly, retroperitoneal processes or pelvic pathology originating in the prostate, ovaries, or urethra may give rise to bilateral hydronephrosis. Patients being evaluated for hydronephrosis should be adequately hydrated, but not overhydrated. Renal ultrasonography may be limited by technical challenges in obtaining the images or be due to inherent characteristics of the test itself. False-negative exams for hydronephrosis occur when there is actual obstruction without calyceal dilation, which is rare unless the kidney was already nonfunctioning. The diagnosis of renal tumors is not within the standard purview of the emergency ultrasonographer. Ultrasound is limited in the identification of renal masses by both their size and sonographic appearance. Expertise and sonographic skill play an important role in accurate identification of renal masses.