Paget’s disease of bone is a focal disorder of unknown etiology characterized initially by excessive resorption and subsequently by excessive formation of bone, culminating in a “mosaic” pattern of lamellar bone associated with extensive local vascularity and increased fibrous tissue in adjacent marrow. In the strict sense this is not a metabolic bone disease, since some portion of the skeleton is spared, no matter how widespread the disease process may be. It is the extraordinary extent of the abnormal remodeling seen in some cases and its metabolic consequences that make necessary a detailed consideration of the disorder in this volume.