Bone densitometry has great potential to improve our understanding of bone development. However, densitometric data in children rarely are interpreted in light of the biological processes they reflect. To strengthen the link between bone densitometry and the physiology of bone development, we review the literature on physiological mechanisms and structural changes determining bone mineral density (BMD). BMD (defined as mass of mineral per unit volume) is analyzed in three levels: in bone material (BMD(material)), in a bone's trabecular and cortical tissue compartments (BMD(compartment)), and in the entire bone (BMD(total)). BMD(material) of the femoral midshaft cortex decreases after birth to a nadir in the first year of life and thereafter increases. In iliac trabecular bone, BMD(material) also increases from infancy to adulthood, reflecting the decrease in bone turnover. BMD(material) cannot be determined with current noninvasive techniques because of insufficient spatial resolution. BM(compartment) of the femoral midshaft cortex decreases in the first months after birth followed by a rapid increase during the next 2 years and slower changes thereafter, reflecting changes in both relative bone volume and BMD(material). Trabecular BMD(compartment) increases in vertebral bodies but not at the distal radius. Quantitative computed tomography (QCT) allows for the determination of both trabecular and cortical BMD(compartment), whereas projectional techniques such as dual-energy X-ray absorptiometry (DXA) can be used only to assess cortical BMD(compartment) of long bone diaphyses. BMD(total) of long bones decreases by about 30% in the first months after birth, reflecting a redistribution of bone tissue from the endocortical to the periosteal surface. In children of school age and in adolescents, changes in BMD(total) are site-specific. There is a marked rise in BMD(total) at locations where relative cortical area increases (metacarpal bones, phalanges, and forearm), but little change at the femoral neck and midshaft. BMD(total) can be measured by QCT at any site of the skeleton, regardless of bone shape. DXA allows the estimation of BMD(total) at skeletal sites, which have an approximately circular cross-section. The system presented here may help to interpret densitometric results in growing subjects on a physiological basis.