Recognition of fibrous dysplasia (FD) as a distinct skeletal disease is commonly attributed to the description of an osteitis fibrosa disseminata occurring in conjunction with various endocrinopathies and skin pigmentation. The bone that is turned over more rapidly in FD is abnormal, qualitatively and quantitatively, in many other critical features ranging from primary modeling to chemical composition, which more directly translate into clinically adverse effects. FD is a disease of excess; abnormal, and imperfect bone growth. The disease produces excess bone growth by causing a localized increase in bone tissue within bone (or local bone mass). The disease causes abnormal bone growth because bone formation does not adhere to the architectural design of the affected, growing bone segments. The territorial definition of cortical bone, cancellous bone, and marrow space is lost, and bone is formed with haphazard trabecular architecture and an irregular internal structure and is mechanically unsound. The disease causes imperfect bone growth because the matrix deposited has an abnormal chemical composition, an abnormal "tricotage," and an abnormal mineral content.