Cryptorchidism, the condition in which one or both testes are not descended fully into the scrotum, is one of the most frequent developmental anomalies of the human male. Although there is spontaneous descent in the first few months of life in many males born with this condition, the prevalence thereafter is about 1%. The genetic direction of testicular descent is only partially understood. There are multiple etiologies of cryptorchidism, both anatomic and hormonal, with many testes having the potential for normal function. Germ cells in the undescended testis fail to undergo normal differentiation from early infancy, hence the recommendation for treatment between 6 and 12Â months of age. Men with undescended testes frequently have impaired sperm production and decreased inhibin B and elevated FSH levels. Only a small portion of men who had unilateral cryptorchidism are infertile based upon paternity; however, in contrast nearly half of men with previous bilateral cryptorchidism are infertile. The risk of developing a testicular tumor, primarily of germ cell origin, is increased substantially, particularly among the bilateral group and those not corrected before puberty.