This chapter provides a detailed overview of the prototype hypophosphatemic disease, XLH. Novel and established clinical features of XLH are reviewed. Key elements central to the pathophysiology of XLH and its related disorders are discussed, including the putative function of the mutated endopeptidase PHEX. The biochemistry of FGF-23 is reviewed with respect to its role in the related disorder, autosomal dominant hypophosphatemic rickets (ADHR). Clinical features of related disorders including ADHR, hereditary hypophosphatemic rickets with hypercalciuria (HHRH), and tumor-induced osteomalacia (TIO) are compared to those observed in XLH. XLH is one of the most common bone diseases seen in children, with available therapies that only partially address the long-term morbidity of the condition. These suboptimal therapeutic approaches require frequent biochemical monitoring, and the panoply of chronic features that complicate the condition persist throughout life. XLH is characterized by renal phosphate wasting leading to hypophosphatemia and low or normal concentrations of 1,25-dihydroxyvitamin D (1,25(OH)2D), an inappropriate response to hypophosphatemia. In children, the disorder first becomes apparent with the development of rickets, skeletal deformities, short stature, and dental abscesses. In adults, manifestations of XLH include osteomalacia, degenerative joint disease, enthesopathy, bone and joint pain, and continued dental disease.