Fibrous dysplasia (FD) is a tumor-like lesion characterized by replacement of normal bone with immature woven bone in a fibroblastic stroma. It arises during skeletal development and it has all the components of normal bone but their differentiation into mature structures is arrested. It may affect one bone (monostotic) or multiple bones (polyostotic). Epidemiology: FD is predominantly found in children and young adults as shown by the age distribution in this graphic. Almost 75% of cases present by age 30 yrs. The highest incidence is between the ages of 3 and 15 yrs. Polyostotic forms usually present by the age of 10 yrs. There is no gender predilection. Presentation: FD is usually asymptomatic and may be discovered incidentally. Patients may present with pain, swelling of bone, deformity or discrepancy in limb length (due to abnormal bone growth), and pathologic fracture (especially when involving femoral neck). Involvement of skull bones (in craniofacial fibrous dysplasia) can cause exophthalmos or cranial nerve compression. When presenting in syndromic settings, other symptoms/signs may be present. Polyostotic fibrous dysplasia manifests at slightly earlier age than monostotic form and may continue into adulthood. Involvement of shoulder and pelvic girdles can cause severe pain, deformities, and fracture. Location: The most commonly involved bones are: monostotic form – ribs (most common), proximal femur, tibia, cranio-facial bones, and humerus; polyostotic form – femur, tibia, pelvis, foot, ribs, and craniofacial bones. Slide courtesy of: Piero Picci, M.D., Director, Laboratory of Experimental Oncology, Instituto Ortopedico Rizzoli, Bologna, Italy. Used with permission.