Osteoporosis is a major public health concern with origins in childhood and is potentially linked to childhood obesity. This study used novel approaches in bone imaging to characterize skeletal development in girls and to assess the influence of whole body and regional soft tissue composition on bone material, structural and geometric properties, the primary determinants of bone strength, controlling for important covariates such as maturation, diet and physical activity. Prospective analyses were conducted to assess associations between measures of total body fat (TBFM) and android fat masses (AFM) and skeletal muscle fat (SMF) content on bone mineral content, density and strength. The results showed that higher TBFM and AFM were inversely associated with changes in cortical bone sites of the femur and tibia. These findings suggest that gains in abdominal adiposity during the pre- and early- pubertal years may contribute to suboptimal bone development and skeletal fragility later in life. The analyses also showed inverse associations between baseline muscle density of the thigh and calf with 2-year changes in bone strength and bone density of the metaphyseal and diaphyseal sites of the femur and tibia. This paradoxical relationship between SMF and bone outcomes was explained by subsequent analyses showing that girls exhibiting larger gains in muscle density experienced larger increases in bone density and strength compared to girls who did not significantly increase muscle density. These findings suggest that fatty infiltration of skeletal muscle contributes to suboptimal bone development in peri-pubertal girls. Further longitudinal analyses were conducted to examine the individual effects of the muscle-bone unit components on 2-year changes in bone strength. These results showed that muscle size contributed to gains in bone strength, independent of its mechanostat effect on BMC. These results underscore the importance of muscle size for promoting bone development and bone strength during growth. A final set of analyses were conducted to examine the effects of dietary fatty acids on bone development. The results of these analyses suggest that while decreasing intakes of AA n-6 FA may benefit bone health, higher intakes n-3 FAs may benefit tibia bone density development in young girls.