Several epidemiological studies have demonstrated a gradual decrease of serum testosterone with aging in men. A considerable number of men will experience hypogonadal androgen levels, defined by the normal range for young men. Thus, in addition to the long-standing use of androgen replacement therapy in the classical forms of primary and secondary hypogonadism, age-associated testosterone deficiency has led to considerable developments in application modes for testosterone. Since oral preparations of testosterone are ineffective, due to the first-pass effect of the liver, or, in case of 17 alpha-alkylation, cause hepatotoxicity, intramuscular injection of long-acting esters, such as testosterone enanthate, have been the mainstay of testosterone therapy. However, the large fluctuations of serum testosterone levels cause unsatisfactory shifts of mood and sexual function in some men; combined with the frequent injections, this delivery mode is thus far from being ideal. In contrast, the transdermal testosterone patches are characterized by favorable pharmacokinetic behavior and have proven to be an effective mode of delivery. Safety data over 10 years indicate no negative effect on the prostate. Nevertheless, the scrotal testosterone patch system is hampered by the application site, which is not easily accepted by many subjects; the non-scrotal patch has a high rate of skin irritations. In view of the drawbacks of the currently available preparations, the most recent developments in testosterone supplementation appear to be highly promising agents. Androgen, which has been available in the United States since mid-2000, will be introduced this year in most European markets as Testogel, a hydroalcoholic gel containing 1% testosterone. Doses of 50-100 mg gel applied once daily on the skin deliver sufficient amounts of testosterone to restore normal hormonal values and to correct the signs and symptoms of hypogonadism. The gel has shown to be very effective and successful in American patients, who have benefited from its availability for almost 3 years. Furthermore, in phase II and III clinical studies, the intramuscular injection of 1000 mg testosterone undecanoate every 12-15 weeks has led to extremely stable serum testosterone levels for a prolonged period of time and has resulted in excellent efficacy. It is very likely in the future that these products will be the mainstay of testosterone supplementation. Whereas the indication for testosterone substitution for men with classical forms of hypogonadism is unequivocal, the use of testosterone in men with age-associated hypogonadism is less uniformly accepted. Yet, the few studies addressing this question indicate that men with testosterone serum levels below the lower normal limit for young adult men and with lack of energy, libido, depressed mood and osteoporosis may benefit from testosterone supplementation. However, it should be kept in mind that the experience documented in studies is limited. Nevertheless, serious side-effects, especially in regard to the prostate, did not occur, with the longest study extending over 3 years.