Patients with acute urologic diseases are frequently seen in the emergency department (ED). Many of these patients will require one or more radiologic studies during the course of their ED evaluations. Traditionally, intravenous pyelography (IVP) has been the diagnostic study of choice, but the emergency physician has a number of other imaging options from which to choose, including formal ultrasonography (US), computed tomography (CT), and contrast angiography. Each of these radiologic tests are performed by a radiologist or radiology technician and interpreted by a radiologist, generating substantial cost to the patient. Another less expensive but effective option is being increasingly used in many EDs; emergency bedside renal US performed and interpreted by the emergency physician. This article reviews the role of formal renal US in the evaluation of acute urologic emergencies, focusing on clinical indications, findings, and relative merits compared with other diagnostic modalities. The last part of this article summarizes the growing body of evidence in support of the use of bedside renal US performed by the emergency physician and reviews the specific technique of performing the US examination.