As men age, testosterone levels decline, and decreased testosterone levels are associated with increased risks of osteoporosis, metabolic syndrome, type 2 diabetes mellitus and mortality. Nevertheless, it is still uncertain whether reduced testosterone level is a cause of ill-health or a marker of pre-existing disease, as systemic illness lowers testosterone levels. Most circulating testosterone is bound to sex-hormone-binding globulin (SHBG) and albumin, whereas a small proportion circulates as free testosterone. Decreased SHBG level is associated with increased risks for insulin resistance and metabolic syndrome, although it would also be expected to be associated with increased free testosterone level. During male aging, total and free testosterone levels fall while SHBG level rises. Thus, associations between decreasing androgens and negative health outcomes might differ across men of various ages. Trials of testosterone therapy report benefits for body composition and BMD, but there are limited data on the effect of testosterone supplementation on cardiovascular risk. Whereas men who have androgen deficiency should be considered for testosterone therapy, the role of testosterone supplementation in older men who are not clearly hypogonadal requires further clarification. Further studies are also needed to establish whether the age-related decline in circulating testosterone level in men can be modified or prevented.