Osteoporosis in men is a significant problem in clinical medicine and in society in general. Systematic consideration of the differential diagnosis of osteopenia and osteoporosis in men is appropriate in every affected patient. Dynamic bone histomorphometry has diagnostic usefulness in some patients and has provided important pathogenetic implications by study of bone remodeling and turnover. Review of the multifactorial pathophysiology of age-related bone loss in men suggests that routine maintenance of adequate calcium and vitamin D intake, exercise, early recognition and treatment of testosterone deficiency, and modification of other osteoporotic risk factors may have prophylactic value. Future basic research on the cellular biology of bone in health and disease and clinical trials assessing the effects of long-term prophylactic and treatment regimens on bone mass and fracture occurrence will expand the understanding of osteoporosis in men.