Male hypogonadism signifies impaired production of testosterone by Leydig cells, or deficient spermatogenesis, but in most clinical disorders both compartments of the testis are abnormal. This is not surprising because extensive biochemical communication occurs between the Leydig cells and the seminiferous tubules (see Chap. 113). Hypogonadism due to pathology intrinsic to the testis (primary testicular failure) or to deficient gonadotropin drive to the testis (hypogonadotropic hypogonadism) may produce similar clinical features, and the tests used to diagnose most forms of hypogonadism overlap. Therefore, a general clinical and laboratory approach to evaluating testicular function will be presented.