The morphology of the skeleton is determined genetically, but its mass and architecture can be adapted by mechanisms sensitive to mechanical loading. Resistance to loading depends on skeletal mass, material properties, geometry and tissue quality. Mechanical loads greater than those habitually encountered effect adaptations in bone, reduce the rate of bone turnover, and activate new bone formation on cortical and trabecular surfaces. Thus, they increase bone strength by beneficial adaptations in the geometric dimensions and material properties of the tissue. Static loads have little role in mechanotransduction and adaptive bone formation is threshold driven and dependent on strain rate, amplitude, and partitioning of the load. That is, only a few cycles of loading are required at any time to elicit an adaptive response, and distributed bouts of loading, are more osteogenic than single sessions of long duration. These parameters of loading are being translated into feasible public health interventions with strong evidence that the structural adaptations persist in to adulthood. When loading exceeds an adaptive threshold, microdamage is initiated that can progress to fatigue, or stress, fractures. Stress fractures (SFx) occur as a result of repetitive non-traumatic cyclic loading, and are common in professional athletes, soldiers and dancers. They account for about 1-7% of all athletic injuries, and also underlie atypical femoral fractures following long-term bisphosphonate therapy. SFx healing progresses with periosteal woven bone stabilising the SFx site (the “stress reaction”), while bone remodelling targets repair of the SFx line. Agents that alter bone material properties (such as mineral homogeneity, collagen cross-linking and toughness) and/or the rate of turnover influence the microdamage propagation and rate of repair, affecting the risk of SFx. Mechanical loading is a potent stimulus to improve bone strength and inhibit bone loss with age, but it has limits and may interact with some therapies to increase SFx risk.