The last decade has seen that several of the dominant and recessive forms of hypoand hyperphosphatemic bone disease have received their molecular explanation. This has led to new insight into the pathophysiology of hypoand hyperphosphatemic bone disease, as well as the understanding of a bone-kidney axis which operates integrated and in parallel with the classical parathyroid-kidney axis in the regulation of phosphorus content in the body. In addition, it has led to the recognition of a Janus face of some of the involved genes, showing both hyperand hypofunction, dependent on the nature of the mutation. In this book chapter, we will present an update on the emerging insight of monogenic hypoand hyperphosphatemic disorders.