Testosterone and synthetic androgens have formerly been used indiscriminately, but are now applied more selectively. They are the only treatment of primary hypogonadism, but are also useful in gonadotropin deficiency and constitutional delay. 17-Alkylated androgens are no longer used. Oral testosterone undecanoate is not suitable for adolescents because of unreliable absorption. The prototype disorder where replacement is necessary is congenital anorchia. As a physiological replacement, an initial dose of 35 mg/m<sub>2</sub> per month for 6 months, followed by 70 mg/m2 for 1 year, and 150 mg/m<sup>2</sup> thereafter, is recommended. No general rules can be given for other types of primary hypogonadism. In testicular atrophy after cryptorchidism, defects of testosterone biosynthesis, galactosemia or other causes, it is advisable to carry out periodic testosterone determinations and to wait until the levels drop below normal. Progress has been made in the treatment of gonadotropin deficiency, and pulsatile gonadotropin-releasing hormone (GnRH) has been shown to be effective in the hypothalamic type. Nevertheless, androgens still have a temporary place in this condition. In constitutional delay of growth and adolescence, treatment is not necessary somatically, but there are often psychosocial reasons. Gonadotropins, GnRH or growth hormone (GH)- releasing hormone have been used. Also treatment with human GH is successful in accelerating height velocity. The most simple and economic treatment is still testosterone in a physiological dose for 3–6 months. Oxandrolone or other synthetic androgens have no advantages.