Urinary tract imaging in the setting of obstruction continues to evolve. Controversy exists because of newly reported imaging techniques, which are not yet universally available or accepted. Previously, the intravenous urogram (IVU) was the accepted standard in evaluation of acute urinary tract obstruction. Today, in most institutions, ultrasound or CT scan have become the initial imaging modality. Ultrasound has assumed a more prominent role in the evaluation of renal obstruction with the emergence of Doppler interrogation to assess renal blood flow, as well as the use of color Doppler imaging to detect urine flow into the bladder. CT scan also has been shown to be effective and efficient in the initial evaluation of flank pain. This article first briefly examines the physiology of acute obstruction as it relates to imaging and then assesses the current utility of each imaging modality in the diagnosis of urinary tract obstruction.