Osteoporosis, a metabolic bone disease characterized by low bone mass, microarchitectural deterioration of bone tissue, and increased susceptibility to fracture, now is recognized to be a disease of men and women. Based on bone mineral density measurements established by the World Health Organization, approximately 2 million men in the United States currently are affected. Information about the pathogenesis, diagnosis, and treatment of osteoporosis in men is accumulating. Estrogen now is known to be necessary for normal skeletal maturation and for maintenance of adult bone mass in men. As we will discuss, the age-related decline in bone mineral density in men seems to result from an age-related decline in the levels of bioavailable estrogen. Osteoporosis candidate genes have been discovered, and their polymorphisms may predispose men to a weakened, osteoporotic phenotype. Determining a bone mineral density standard to diagnose men with osteoporosis has proven challenging, often leaving diagnosis until a fragility fracture has occurred. General prevention strategies for osteoporosis in men are targeted at men most likely to sustain a fragility fracture. Specific medical therapies for osteoporosis in men, including bisphosphonates and intermittent parathyroid hormone, are proving promising.