Paget disease of bone is a common disorder affecting approximately 3%-4% of the population over 40 years of age. The pathologic abnormality in Paget disease is excessive and abnormal remodeling of bone. Three pathologic phases have been described: the lytic phase (incipient-active), in which osteoclasts predominate; the mixed phase (active), in which osteoblasts cause repair superimposed on the resorption; and the blastic phase (late-inactive) in which osteoblasts predominate. Radiographic appearance of Paget disease reflects these pathologic changes and is often characteristic. Initially, there is osteolysis, particularly affecting the skull (osteoporosis circumscripta) and subchondral long bones, with subsequent development of trabecular and cortical thickening and enlargement of bone in the mixed phase of the disease. Finally, areas of sclerosis may develop in the blastic phase. Frequent sites of involvement include the skull (25%-65% of cases), spine (30%-75%), pelvis (30%-75%), and proximal long bones (25%-30%). Bone scintigraphy typically demonstrates marked increased uptake of radionuclide in all phases of Paget disease. Computed tomography and magnetic resonance imaging often show changes similar to those seen radiographically in noncomplicated Paget disease with maintenance of yellow marrow. Complications of Paget disease include the effects of osseous weakening (deformity and fracture), arthritis, neurologic symptoms, and neoplastic involvement. Sarcomatous transformation is the most feared complication, occurring in approximately 1% of cases, and is seen on images as focal bone destruction extending through the cortex with an associated soft-tissue mass. Recognition of the radiologic spectrum of the appearances of Paget disease usually allows prospective diagnosis and differentiation of its associated complications, which helps guide therapy and improve patient management.