Bone mass is a function of bone size and volumetric bone mineral density (BMD) and is a key determinant of bone strength. The ultimate test of any measurement of bone mass is its ability to predict clinical bone fragility. Bone strength reflects the integration of two main features: BMD and bone quality. Bone quality refers to bone architecture, turnover, damage accumulation, and mineralization. A variety of noninvasive methods are available to assess the peripheral, central, or entire skeleton. The vast majority of studies that assessed bone acquisition in neonates, infants, and children were based on DXA measures of bone mineral content (BMC), projected bone area, and areal bone mineral density. Bone structural strength is determined not only by BMD, but also by the geometry of the bone. Bone size and the distribution of bone mass influence the resistance of bone to fracture. Cortical bone mass is distributed about a central axis to provide maximum strength with a minimum of material.