Serum testosterone levels decline in men with increasing age. Late-onset hypogonadism with its characteristic symptoms can occur in men as they age. Typical symptoms of late-onset hypogonadism are decreased libido and sexual function, osteoporosis, altered distribution of body fat, overall reduction in physical strength, and alterations in general mood. Late-onset hypogonadism can be treated with testosterone, and different forms of testosterone have become available for this indication. The aim of testosterone replacement therapy is to produce serum testosterone levels within the physiological range avoiding levels above and below this range. Although hormone replacement therapy has become accepted in aging males, careful consideration of the indications and therapy monitoring are still required since there are major concerns about the long-term outcome of this therapy and particularly its effects on the prostate gland.