Fibroblast growth factor 23 (FGF23) is a recently characterized peptide hormone produced mainly in the bone. It is secreted in response to dietary phosphorus load, and its main function is the promotion of urinary phosphate excretion and the suppression of active vitamin D (1,25D) production in the kidney. As such, FGF23 plays an important role in the maintenance of systemic phosphate homeostasis. In the advanced stages of chronic kidney disease (CKD), the kidney cannot excrete a phosphate load even in the presence of high levels of FGF23. This results in both hyperphosphatemia and a low level of 1,25D that stimulates the secretion of parathyroid hormone (PTH), leading to the development of secondary hyperparathyroidism. In chronic dialysis patients without residual renal function, FGF23 levels become extremely high due to stimulation by vitamin D therapy as well as by high levels of serum phosphate and PTH. Recent investigations have demonstrated that serum FGF23 level can be a useful marker for the prediction of the future development of refractory hyperparathyroidism and the response to vitamin D therapy in dialysis patients. In addition, putative protective roles of FGF23 against calcification have also been speculated on. Further elucidation of the mechanisms of FGF23 action will be needed to understand the various roles of FGF23 in CKD-Mineral and Bone Disorder (CKD-MBD).