Because estrogen (E) and T are the major sex steroids in women and men, respectively, the traditional view had been that E primarily regulated bone turnover in women and T played the analogous role in men. The description of ER- deficient and aromatase-deficient males, however, initiated a major shift in our thinking on the relative roles of T and E in regulating the male skeleton, because these individuals all had unfused epiphyses, high bone turnover, and osteopenia. Similar, albeit less striking, findings were noted in mouse models with knock-out of either the ER-α or the aromatase genes. Although these human experiments of nature and mouse knock-out models clearly demonstrated an important role for E in the growth and maturation of the male skeleton, they did not define the role of E vs. T in regulating the adult male skeleton. The past several years have witnessed an accumulation of evidence from observational as well as direct interventional studies that now clearly indicates that E plays a major, and likely dominant, role in bone metabolism in men. These data also suggest that a threshold level of bioavailable (or non-SHBG bound) E is needed for skeletal E sufficiency in the male, and that with aging, an increasing percentage of elderly men begin to fall below this level. It is this subset of men who may be at greatest risk for the development of age-related bone loss and osteoporosis. Moreover, these men may also be the ones most likely to respond favorably to treatment with selective E receptor modulators, or perhaps even to T replacement, because the skeletal effects of the latter may be mediated largely via aromatization to E.