This chapter reviews the conceptual basis for understanding the relationship between mechanical loading and skeletal adaptation, along with the specific effect of exercise on bone mass in men. Complete understanding of the relationship of exercise to bone mass in humans must await development and validation of accurate, quantitative estimates of mechanical loading history. Although this has not yet been accomplished, sufficient information is available to permit general conclusions, as well as speculation about the kinds of exercises that are likely to prove most osteogenic. Cross-sectional studies provide support for the notion that habitual athletic endeavor promotes superior bone density in men compared with that of a sedentary life-style. The magnitude of this difference is likely to depend on the nature of the activity, the age at which it was initiated, and the number of years spent in training. Physical activity may enhance peak bone mass attained if initiated before the age of 20. Muscle mass and strength is likely to contribute positively to bone mineral density (BMD) at this age, as well as during later years. Bone displays a clear site specificity for mechanical load-induced adaptation at all ages.