Fibroblast growth factor 23 (FGF‐23) is likely to be the most important regulator of phosphate homeostasis, which mediates its functions through FGF receptors and the coreceptor Klotho. Besides reducing expression of the sodium‐phosphate cotransporters NPT2a and NPT2c in the proximal tubules, FGF‐23 inhibits the renal 1α‐hydroxylase and stimulates the 24‐hydroxylase, and it appears to reduce parathyroid hormone (PTH) secretion in short‐term studies. FGF‐23 synthesis and secretion by osteocytes and osteoblasts is upregulated through 1,25‐dihydroxyvitamin D₃ [1,25(OH)₂D₃] and through an increased dietary phosphate intake. FGF‐23 levels are elevated or inappropriately normal in patients with tumor‐induced osteomalacia and several inherited hypophosphatemic disorders, but the most significant increases are found in patients with chronic kidney disease (CKD). During the early stages of CKD, increased FGF‐23 production enhances urinary phosphate excretion and thus prevents the development of hyperphosphatemia, reduces the circulating levels of 1,25(OH)₂D₃, and therefore contributes to the development of secondary hyperparathyroidism. In patients with end‐stage renal disease (ESRD), FGF‐23 levels can be extremely high and were shown to be predictors of bone mineralization, left ventricular hypertrophy, vascular calcification, and mortality. It remains to be determined, however, whether FGF‐23 represents simply a sensitive biomarker of an abnormal phosphate homeostasis or has, independent of serum phosphate levels, potentially negative “off‐target” effects. Nonetheless, reducing the production and/or the biologic activity of FGF‐23 may be an important therapeutic goal for this patient population. © 2010 American Society for Bone and Mineral Research.