Bone mineralisation is the process by which calcium and phosphorus salts, mainly hydroxyapatite, are deposited into the collagen matrix of bone, giving the skeleton its hardness and load-bearing strength. It depends on adequate vitamin D to support calcium absorption from the gut, sufficient dietary calcium and phosphorus, and balanced activity of bone-building and bone-resorbing cells. Impaired mineralisation in children causes rickets, with bowed legs and stunted growth, while in adults it causes osteomalacia, with bone pain and weakness. Fortification of milk, cereals, and plant-based drinks with vitamin D and calcium is a key food-policy tool against deficiency.