Testosterone supplementation can help reduce many of the symptoms associated with androgen deficiency in the aging male by its effects on various parts of the body. Bone mineral density can decrease in the hypogonadal man and this may contribute to the increased fracture rate in the elderly. Testosterone therapy can improve bone mineral density and bone architecture by increasing bone formation and decreasing bone resorption - the possible benefits on fracture rate are unknown. Testosterone also improves body composition by reducing body fat mass and increasing lean body mass, and by increasing epidermal thickness, but its effects on muscle strength are still debated. In patients with diabetes and androgen deficiency, testosterone supplementation appears to reduce blood glucose and this could have important implications for cardiovascular risk reduction in patients with diabetes or the metabolic syndrome. The wide-ranging benefits of testosterone therapy in young and old men are clear and it appears that the route of administration (intramuscular, oral, or transdermal) does not alter this fact, but future work could illustrate even more profound effects of testosterone (e.g., in reducing cardiovascular risk) that could result in its recommended use in a wider range of patients.