The male hormone and sperm are produced by the testicle. An undescended testicle occurs when the testicle cannot be found in its normal position in the scrotum. The testis normally migrates down from high in the abdomen and groin to take its normal position in the scrotum before a child is born. Up to 4% of term infants have an undescended testicle, with a higher incidence in premature males. Spontaneous descent into the scrotum occurs in three-fourths of undescended testicles during the first three months of life. Most undescended testicles are palpable in the groin, but some are nonpalpable, being intra-abdominal or absent. Retractile testicles are not truly undescended but withdraw into a position outside of the scrotum because of a hyperactive cremasteric reflex. Retractile testicles can be manipulated into the scrotum, but some go on to become undescended as the child grows. Treatment for the undescended testicle can be undertaken any time after 6-9 months of age. Relocating the testis into the scrotum may decrease the risk of fertility problems, particularly if done at an early age. There are other advantages to a location within the scrotum, including a cosmetic advantage and alleviation of any psychological effects resulting from an empty scrotum. The scrotal testis may be less amenable to injury than a testis outside the scrotum. A testis that has not made it into the scrotum is not accessible to physical examination and is at increased risk for testicular cancer, the most common solid tumour in men up to age 40 years. Testicular carcinoma is highly curable when detected early, and monthly self-examination is the best way to do this. Treatment for an undescended testicle may involve hormonal therapy, although the success rate is below 10% when retractile testicles are excluded from the treatment groups. The surgical treatment for an undescended testicle is called an orchiopexy. Palpable undescended testicles are found through a small inguinal skin incision. Nonpalpable undescended testicles require surgical exploration, and this is done by diagnostic laparoscopy. Most of these children probably had torsion or twisting of the testis on its blood supply prior to birth that led to the small testis. When a boy is left with a single functioning testis, we may recommend anchoring it to minimize the chances of losing it to torsion later in life.