Renal US is one of several imaging modalities available to the emergency physician in the evaluation of patients with acute urologic disorders. It offers excellent anatomic detail without exposure to radiation or contrast agents but does not assess renal function. It is particularly useful in the evaluation of renal colic, although its role here may decrease with increasing availability of helical CT. It also has an important role in excluding bilateral renal obstruction as the cause of acute renal failure. Doppler renal US is likely to take on a more prominent role in the evaluation of renal trauma and is the diagnostic study of choice to rule out renal vein thrombosis. Bedside emergency renal US performed and interpreted by emergency physicians with limited training and experience is gaining in use and acceptance. Its role at present is primarily to identify unilateral hydronephrosis in patients with suspected renal colic. This role is likely to expand in the future as emergency US use grows and technology advances. Bedside emergency renal US may eventually be used in the evaluation of patients with acute renal failure, suspected renal vein thrombosis, and renal trauma.