Since the advent of US, nearly every malformation of the urinary tract has been diagnosed in utero. Physiologic dilatation of the renal pelvis occurs and can be misdiagnosed as hydronephrosis, so the normal gestational development of the urinary tract, and its visualization by ultrasound, should be understood. Gestational diagnosis is important for family counseling, collaboration of the perinatal team, and for determining both the future evaluation needs and the possibility of intervention. Those infants at high risk, as well as others who have specific signs and symptoms of possible urinary tract disease, deserve prompt and careful postnatal imaging studies. Ultrasonograms may be combined with other imaging modalities in specific circumstances to provide accurate diagnoses and some functional information.