Paget’s disease, or Morbus Paget (named after Sir James Paget, who first accurately described the disease in 1877), is also called osteodystrophia deformans or osteitis deformans. It is a localised non-inflammatory disease of bone caused by uncontrolled, increased bone resorption, by pathological, large multinucleated osteoclasts (Fig. 56.1), which in turn induces disorganised bone formation. This disease illustrates graphically what happens when there is complete local deregulation of osteoclasts together with partial abrogation of “coupling” in the osteological meaning of the term. Osteoblasts are stimulated to replace the resorbed bone, but the osteoid is randomly produced and not laid down as lamellae, so that the resulting bone is dense but mechanically inadequate. The focal greatly increased bone turnover is accompanied by hypervascularisation and increased blood flow. Deformities of the effected bones are the rule. Mono- and polyostotic forms of Paget’s disease are recognised.