The authors describe the diagnostic procedures for testicular localization in patients with non palpable undescended testes. They consider the procedures as invasive and non invasive. The first procedures include ultrasonography, C.T. and N.M.R., the second ones laparoscopy, selective arteriography and venography of the spermatic vessels. Anatomy of spermatic venous system and catheterization techniques are described. Conclusions are that when non invasive procedures fail to localize the testis, gonadal venography is the procedure of choice for the correct diagnosis. Preoperative testicular localization often reduces the extent of surgical exploration and the anesthesia time. These patients have to undergo surgical treatment also when gonadal venography shows a blinding end vein in the inguinal canal, because of the intra-abdominal testis possible degeneration.