Ultrasonography is the first imaging approach in evaluation of patients with urinary obstruction. Presence of hydronephrosis, urinomas and inflammatory complications can be assessed. Moreover, the level and the cause of obstruction are often identified. The major limits of ultrasonography are poor visibility of the lumbar portion of the urinary tract and lack of specificity in detection of pyelocaliectasis. These limits are partially overcome with Doppler studies. Several factors can bias diagnostic accuracy of colour Doppler ultrasonography in evaluating renal obstruction; in particular, partial or intermittent obstruction cause minimum hemodynamic changes in patients with renal colic. Sensitivity of colour Doppler ultrasonography to detect obstruction decreases following medical treatment with NSAID and increases following hydration and diuretics administration. Diuretic ultrasound, colour Doppler evaluation of ureteral jets and evaluation of the twinkling artefact of stones are useful imaging techniques as well. Tissue harmonic imaging is a new imaging technique available in most latest generation US equipments. Evaluation of the kidney and of the urinary tract improves using this technique due to artefact reduction and increased contrast resolution.