The testis is involved in male sexual differentiation during prenatal life, development of secondary sexual characteristics at puberty and reproduction and sexuality in adults. The testis has two functions, testosterone secretion and spermatogenesis. Testicular failures have different clinical presentations according to age and the relative extend of the decrease of testosterone secretion and sperm production. Diagnosis might be done at birth, because of ambiguous genitalia, micropenis, hypospadias, cryptorchidism; in teenagers because of delayed puberty; in adults because of sexual problems and regression of sexual characteristics or infertility. The increase of gonadropin levels indicates the testicular origin of hypogonadism. Since spermatogenic failure was often more severe than endocrine defect, FSH was often more elevated than LH. The causes of testicular failure are numerous, the description at the molecular level of genetic causes is in progress. Intracytoplasmic sperm injection greatly enhanced the possibility of conception.