Aging of male mammals is a very recent evolutionary event observed mostly in humans and animals in captivity. Most animal species in the wild do not live beyond their reproductive years; during periods of food deprivation, many small animals may not even live beyond puberty. Even among humans, only the men and women of the past two generations have enjoyed a life expectancy of greater than fifty years. With increasing life expectancies on all continents except Africa, the reproductive problems of aging men and women have begun to receive the attention that they deserve. Aging of humans is associated with functional alterations at all levels of the reproductive axis that affect both the steroidogenic and gametogenic compartments. Even after forty years of investigation, the controversy surrounding the use of hormone replacement in postmenopausal women has grown only louder (1-4); in contrast, the issue of testosterone replacement in older men has been shrouded in acerbic debate from its very inception, even though not a single, adequately powered, long term, randomized trial of testosterone replacement has yet been conducted. There is agreement that in young men with classical hypogonadism, physiologic testosterone replacement is relatively safe and has many beneficial effects in improving lean body mass, sexual function, energy, mood, and sense of well being and reducing fat mass (5-8). However, the data from otherwise healthy, young, men with classical hypogonadism should not be directly extrapolated to older men with age-related decline in serum testosterone concentrations (9-10). As discussed in this chapter, there is a growing body of evidence that serum testosterone levels decline as a function of age; however, the effects of testosterone supplementation on health-related outcomes in older men have not been studied. While short term administration of testosterone replacement in older men has been associated with a relatively low frequency of adverse effects, long term data on the effects of testosterone supplementation on the risk of prostate disease and cardiovascular events are lacking. Thus, the risks and benefits of long term testosterone replacement in older men remain unknown.