Mechanical stimuli are extremely important to the development, growth, and aging of the skeleton. Long bone cross-sectional geometry results from the response and adaptation of the skeleton to dynamic mechanical loading, including bending, torsion and axial compression. A computer model was developed to simulate the interaction of intrinsic biological and extrinsic mechanical factors in the development of the cross-sectional morphology of long bones. The model incorporates a strong influence of biologically induced bone formation during early development. In addition, age-related changes in body weight and muscle mass were included in the form of an assumed mechanical loading history during growth and development. Based on the bone stress stimulus generated by the applied loads, mechanically induced apposition and resorption rates were calculated at the periosteal and endosteal surfaces using a previously developed bone modeling theory. These methods successfully emulated the growth-related changes seen in long bone diaphyseal structure as well as changes observed in mature bones during aging. These ideas were further tested by examining perturbations in development, growth, and adaptation in humans and other species. When development under and adaptation to altered mechanical loading conditions were modeled, the results reproduced the main features present in vivo and in vitro under similar conditions. Simulation of skeletal morphogenesis across a range of animal sizes produced adult long bone geometric scaling relationships close to those measured in vivo. Experiments were performed to examine the effect of reduced loading on the growth of the rat femur. Significant differences in femoral structural behavior were measured in torsion tests to failure and these changes were explained by concomitant mid-diaphyseal geometric changes. Using these data and the models developed earlier, normal growth and functional adaptation were successfully modeled in the rat. The results of this work indicate that long bone development, growth and adaptation are strongly influenced by dynamic mechanical loading. The fundamental ontogenetic processes occurring from embryogenesis through aging are similar and can be theoretically modeled.