The aging process in men is accompanied by a progressive decline in serum testosterone (T) levels. The magnitude of such decline and the prevalence of older men with a clinically relevant reduction of T levels remains controversial despite a relatively large number of studies. The extent to which androgen decline leads to health problems that might affect or alter the quality of life is under debate. The validity of common methods used for assessment of testosterone levels has been recently questioned. In addition, total testosterone levels may be misleading because of an increase in sex hormone-binding globulin levels. In addition to chronological aging, the various illnesses occurring in mid- to lateadult life and the medications used to treat them further contribute to lowering circulating testosterone levels. Testosterone replacement therapy has been advocated for ameliorating sexual dysfunction and resistance to pro-erectile drugs in men with reduced testosterone levels. Independent meta-analyses showed that the effects of T on erectile function and libido, are inversely related to the mean baseline T concentration. These reviews show that T treatment might be useful for improving erectile dysfunction in selected subjects with low or low-normal T levels. The evidence for a beneficial effect of T treatment on erectile function should be tempered with the caveats that the effect tends to decline over time, is progressively smaller with increasing baseline T levels, and long-term safety data are not available. Recent studies have also demonstrated that testosterone replacement in hypogonadal men improves bone density, angina and possibly the metabolic syndrome. Nevertheless, these fascinating data still need large-scale, long-term, randomized controlled trials to formally confirm the efficacy of T replacement in symptomatic middle-aged and elderly men with reduced T levels.