Since the identification of fibroblast growth factor 23 (FGF-23) by positional cloning in 2000 as the gene implicated in autosomal dominant hypophosphataemic rickets (ADHR), subsequent research had shown that several forms of hypophosphataemic rickets are associated with elevated levels of FGF-23 [1]. A glycoprotein of 251 amino acids, it is classified as a phosphatonin that is secreted almost exclusively by the osteoblasts and osteocytes. Its N-terminal end binds to FGF R1c, while the C-terminal portion binds to the transmembrane protein co-factor klotho; both these interactions are considered essential for the phosphaturic action of FGF-23 2, 3. FGF 23 causes hypophosphatemia by down-regulating the Na-Pi 2a and 2c in the proximal tubule of the kidney [4]. While phosphate and 1,25(OH)2 vitamin D are positive regulators of FGF-23, an excess of FGF-23 causes hypophoshphatemia and suppressed levels of active vitamin D [5]. This review will focus of the various causes of inherited FGF 23 excess, their clinical presentation, evaluation and management.