Congenital hydronephrosis is an abnormal dilatation of the renal collecting system that is present in fetal life. The International Fetal Urological Association defines a separation of more than 0.5 cm of the renal collecting system before 24 weeks of fetal age and a separation of more than 1 cm after 24 weeks and in the neonatal period as the diagnostic criteria for hydronephrosis. Currently, with the popularity of ultrasound, more and more fetuses and newborns are found to have different degrees of hydronephrosis. And young parents often don’t know how to deal with this situation when they encounter it. The etiology of congenital hydronephrosis is complex, and it is useful to understand the anatomy of the human urinary system before understanding its pathogenesis. The kidneys are connected to the bladder through the ureter, and the bladder is connected to the outside of the body through the urethra. The urine produced by the kidneys is discharged from the body along the ureter, bladder and urethra. In this regard, poor drainage anywhere in the ureter, bladder, and urethra can lead to hydronephrosis. The common understanding is that the downstream is not open leading to upstream fluid accumulation. Professionally, the etiology of hydronephrosis can be divided into obstructive and non-obstructive hydronephrosis, the former including downstream (ureter, bladder, urethra) strictures due to various causes. The latter includes vesicoureteral reflux, physiological dilatation of the renal pelvis, etc. The etiology is different and the treatment modalities are very different. Therefore, young parents who encounter hydronephrosis must first find out the specific cause before they can prescribe the right medicine. Among all the causes, pelvic ureteral junction stenosis is the most common, accounting for more than 85% of hydronephrosis. It is more common in men than in women and in the left side than in the right side. Stenosis of the ureteropelvic junction means that urine is obstructed at the exit from the renal pelvis, leading to hydronephrosis. Since the renal pelvis of young children is small, even in adults it does not exceed 10 ml, the narrowing of the ureteropelvic junction prevents urine from entering the ureter smoothly and collects in the pelvis. Over time, this can lead to dilatation of the renal pelvis. In children with mild obstruction, the smooth muscle of the renal pelvis is enlarged, peristalsis is enhanced, and urine can slowly drain into the ureter, so hydronephrosis can enter a relatively stable state and develop very slowly, and renal function is not greatly affected. These children show no significant enlargement of hydronephrosis at regular follow-up ultrasound, and no obvious symptoms are desirable. There is no significant increase in hydronephrosis even in adulthood. This group of children often does not need urgent surgical treatment, but only regular follow-up observation. In the case of severe obstruction, the fluid gradually increases, the renal pelvis and calyces gradually expand, and even compress the renal parenchyma to impair renal function. These patients need early surgical intervention to relieve the obstruction, otherwise the kidney function may suffer serious damage or even loss of function in a short period of time. Therefore, the timing of surgery is not limited by age in this group of children. In children with pelvic ureteral junction stenosis, the most common surgical approach is dissecting pyeloureteroplasty, which is commonly understood as the removal of the stenosis and re-suturing of the canal before and after the stenosis to allow urinary drainage. Many hospitals can now perform this type of surgery laparoscopically, making it even less invasive and allowing for a more rapid recovery. After surgery, there is no significant resistance to urine entering the ureter in the renal pelvis, and renal function can be restored to some extent. Children with early mild hydronephrosis may even return to normal. However, most of the dilated renal pelvis and the thickness of the renal parenchyma cannot be restored to normal. The recovery is most obvious at 6 months after surgery, and is basically fixed 1 year after surgery.