Paget’s disease of bone (PDB) is a focal disease of bone remodelling. It is usually diagnosed in older adults. The precise causes of PDB remain incompletely understood. There seems to be a strong genetic component, but environmental factors appear likely to be important in triggering disease in susceptible individuals. Approaches to diagnosis are set out using the methods outlined in Chapter 2 and consideration of the basic biological mechanisms that underlie lesion formation discussed in Chapter 3. Bones affected by PDB pass through three broad phases and guidance on diagnosis of the various phases of PDB using macroscopic, radiographic and microscopic techniques provided. Consideration is given to the ways in which paleopathological investigations can help our understanding of how PBD may have changed over time and assist in evaluating the role of possible risk factors. A series of bullet points are used to summarise the core concepts covered.