Ureteropelvic junction obstruction (UPJO) is the most frequent cause of dilatation of the upper urinary tract in neonates. The most important aim of the diagnostics and treatment is the prevention of progressive irreversible renal damage and the subsequent sequelae. The widespread and easy use of ultrasound has led to a significantly increased detection rate of dilatations of the urinary tract, especially prenatal. The current guidelines recommend as postnatal examinations ultrasound followed when necessary by dynamic radioisotope renography, depending on the extent of the dilatation. In special cases, voiding cystourethrography (VCUG), contrast-enhanced voiding urosonography (ceVUS) or functional magnetic resonance urography is also indicated; however, the challenge is not only to decide which children can be observed conservatively and which need surgery, but also which children need which of these imaging modalities and how to monitor them by morphological imaging during follow-up.