Cryptorchidism or undescended testis (UDT) is one of the most common pediatric disorders of the male endocrine glands and the most common genital disorder at birth. 1. Overview Cryptorchidism or undescended testes (UDT) is one of the most common pediatric disorders of the male endocrine glands and the most common genital disorder at birth. The main causes of treating cryptorchidism include impaired fertility potential, testicular malignancy, torsion and/or an associated increased risk of inguinal hernia. Over the past half century, there have been significant changes in the diagnosis and treatment of cryptorchidism. The current standard of care in the United States is testicular fixation, in which the testis is fixed in the scrotum, and hormonal therapy is less often advocated. However, for susceptible individuals, even if the testis is successfully repositioned in the scrotum, this approach may reduce, but not completely prevent, the occurrence of these potential long-term sequelae. 2. Definition of cryptorchidism (1) Cryptorchidism, or undescended testis, where the testis fails to descend into the scrotum, usually refers to a testis that is already present but located outside the scrotum, but may also end up with a confirmed missing testis. The term absent testis usually refers to a process in which the testis is or has been atrophied and there is evidence of its initial presence, but it is likely to disappear during development due to vascular distortion or other vascular factors. (2) Congenital cryptorchidism refers to a testicle that is located outside the scrotum from birth. Acquired cryptorchidism is a condition in which the testicle is located inside the scrotum at birth, but is later found to be located outside the scrotum. The specific location of the cryptorchid can be anterior to the scrotum (at or above the entrance to the scrotum), superficial inguinal pouch (distal or lateral to the opening of the external inguinal ring, anterior to the rectus abdominis muscle), external inguinal ring (or anterior to the pubic bone), inguinal canal, or even ectopic to the perineum (most common), or peritoneal cavity (internal inguinal canal ring, near the bladder, iliac vessels, or kidneys). (3) Acquired cryptorchidism refers to an ascending testicular position, i.e., a spontaneous and marked change in the position of the testis from within the scrotum to outside the scrotum sometime after birth, and also includes such changes in testicular position induced by inguinal surgery. Retractile testis is a state in which the testis is initially located outside the scrotum on examination or easily and spontaneously leaves the scrotum (usually associated with a violent testicular reflex), but can be repositioned by manipulation and remains, at least temporarily, tension-free and stable in the scrotum. Atrophic testis is a significant reduction in testicular size that occurs in the inguinal region or after testicular surgery, and also includes a reduction in testicular size due to prolonged extra-scrotal exposure or primary dysplasia. 3. Complications of cryptorchidism (1) Decreased fertility or sterility Cryptorchidism is the main factor of male infertility and probably the most dangerous one. The pathological change of cryptorchidism is the obstruction of germ cell development, so it will lead to reduced fertility or infertility. Generally speaking, patients with bilateral cryptorchidism have a significant decrease in fertility. The fertility of unilateral cryptorchidism is related to the degree of development of germ cells and epididymis in the contralateral testis in addition to the testicular position. (2) Congenital inguinal hernia According to the literature, about 65% of patients with cryptorchidism have congenital inguinal hernia in combination. The reason for this is that in the majority of patients with cryptorchidism, the sphincter duct is not closed and the intestinal collaterals descend into the sphincter cavity in the scrotum. In general, we can wait until the child is a little older and treat the hiatal hernia together with the cryptorchidism surgically. (3) Testicular torsion Statistics show that the incidence of testicular torsion in cryptorchidism is 21-53 times higher than that of testicular torsion in the scrotum; it may be related to the abnormal attachment of the testicular lead or the levator muscle. Cryptorchid testicular torsion generally presents as a painful inguinal mass with symptoms similar to those of inguinal hernia, but usually without digestive symptoms. The testicular torsion on the right side should be distinguished from acute appendicitis. (4) Testicular injury The cryptorchid is often located in the inguinal canal or near the pubic symphysis, and its location is superficial and fixed, and it is easy to be traumatized by external violence. After the injury, the testis will undergo fibrous degeneration, which will accelerate its atrophy and lead to spermatogenic disorders and changes in sexual function. (5) Testicular malignancy The chance of testicular tumor in patients with cryptorchidism is 21~53 times higher than that of normal people. The incidence of malignancy in high cryptorchidism, especially intra-abdominal cryptorchidism, is higher, and its malignancy rate is 4-6 times higher than that of low cryptorchidism. The age of onset of cryptorchidism malignancy is around 30 years old. The possible causes of cryptorchid malignancy are related to local temperature, blood flow disorder and endocrine changes, in addition to self factors. In addition to early repositioning and fixation, cryptorchidism should be followed up for life after surgery, and attention should be paid to the development of testicular tumor on the opposite side.