Bone size, shape and internal architecture, and not just bone mass, account for differences in bone strength between individuals, sexes and races. The differences in bone morphology in old age - whether an individual's bone size and mass occupy the 5th, 50th or 95th percentile - is determined early in life. Bone traits track from the position established early in life. Genetic and environmental factors establish the morphological features of bone through the cellular machinery of bone modelling and remodelling which adapts bone to its loading circumstance by modifying its size and shape and the distribution of its mass. The need for both strength for loading and lightness for mobility are achieved by deposition of bone where it is needed and removal of bone from where it is not. The machinery has enormous capacity during growth, as can be seen in the bone structure of the elite athlete, but not during advancing age because of changes in the cellular machinery itself and in systemic hormonal regulatory factors.