Rickets/osteomalacia is a disorder causing mineralization defect and bone and skeletal fragility, although production of bone matrix proteins and their architecture is not impaired. The disease is called rickets and osteomalacia in children during skeletal development and in adults, respectively. Pathophysiology in rickets/osteomalacia is defect in vitamin D actions and/or hypophosphatemia. Vitamin D deficiency, inability of activation of vitamin D in vivo or functional derangement in vitamin D receptor is involved in impaired actions of vitamin D. Common causes of hypophosphatemia are excessive actions of fibroblast growth factor (FGF) 23 and renal tubular dysfunction. Among them FGF23 could be a principal regulator for phosphate metabolism, and many investigators are engaged in exploration of physiological and pathophysiological roles of FGF23 in human.