Osteoporosis is one of the most common and debilitating diseases of postmenopausal women. Recent advances in understanding the bone remodeling unit have clarified the pathophysiologic processes that contribute to bone loss after the onset of estrogen deprivation. Epidemiologic studies have suggested a protective effect from long-term estrogen replacement therapy on fracture risk. This article examines the key role estrogens play in bone remodeling and the current evidence that estrogen treatment in postmenopausal women reduces the likelihood of osteoporotic fractures.