Although urinary tract obstruction is an age old problem with IVU the time-honored imaging technique of choice, recent advances in renal ultrasonography and abdominal and pelvic CT scanning have advanced these alternative modalities to the forefront. No rigid algorithm for renal obstruction is recommended, because the utilization of these techniques is in a state of flux. The use of one modality over another as a first-line imaging technique likely will depend on the equipment availability and expertise of a given institution. The choice also may be guided by each institution's cost of each type of examination. Although a role for the IVU remains in the evaluation of acute renal obstruction, ultrasonography and CT scan have emerged as alternative primary screening tools. Undoubtedly, the future will bring further changes in our assessment of renal obstruction. The cost effectiveness and definitiveness of evaluation likely will become more important driving factors than they have been in the past. The radiologist and urologist together must assess available imaging techniques, their effectiveness, and the institution's expertise to develop a cost-effective and efficient system for assessment of renal obstruction.