The sites of obstruction found during development are reviewed: high (ureteropelvic junction), mid (ureterovesical junction) and low (posterior urethral valves). Diagnostic evaluation of the fetal urinary tract is reviewed. The management of obstructive uropathies is briefly outlined and centres around the level of the lesion and whether the obstruction is unilateral or bileteral. Antenatal intervention is indicated whenever the function of both kidneys is threatened. The amount of amniotic fluid remains a good guide to renal function. The role of fetal urinary electrolytes as a prognostic indicator is not yet clarified.