The testicles develop before birth in the abdominal cavity. Normally, they descend into the scrotum before birth. Undescended testicle occurs when one or both testicles fail to move into the scrotum prior to birth.

Undescended testicles are fairly common in premature infants, and occur in about 3 to 4% of full-term infants. About 65% of the testicles typically descend by 9 months of age.

Once a testicle has been discovered in the scrotum, it is generally considered descended, even if it is temporarily retracted on a later examination. Sometimes a condition (referred to as retractile testes) will develop, in which the examiner can sometimes locate the testicles and sometimes not.

This occurs because of the strength of the muscular reflex (cremasteric reflex) that retracts the testicles and the small mass of the testicles before puberty. In this instance, the testicles descend at puberty. These testicles are considered a variant of normal and do not need surgical correction.

Testicles that do not descend by one year of age should be carefully evaluated. The literature suggest that definitive surgery should be accomplished by three years of age to confirm diagnosis and to lessen the likelihood of permanent testicular damage.

Testicles that do not naturally descend into the scrotum are considered abnormal throughout the patient's life. These undescended testicles have an increased likelihood of developing cancer regardless of whether they are brought down into the scrotum or not.

Bringing the testicle into the scrotum maximizes sperm production and thus increases the odds of good fertility. It also allows examination for early detection of testicular cancer.

In other cases, such as vanished testis, no testicle may be found upon surgical exploration. This may be due to in utero vascular insult (testicular torsion), resulting in an infarction (death of affected tissue).

Alternatively, this may be associated with congenital anomaly, in which case blind ending blood vessels are seen in the vas deferens (the tube which normally carries sperm).