Bone-forming tumors are defined by neoplastic cells that differentiate along the lines of osteoblasts that deposit neoplastic bone. The morphology and biological spectrum of bone-forming tumors is broad, and their accurate diagnosis requires the careful correlation of their clinical, morphologic, and radiologic characteristics. Immunohistochemical and molecular analyses have an important role in select instances. At present, the identification of neoplastic bone largely depends on histologic analysis, which can be subjective. The major types of osteosarcoma are defined according to their morphology, origin within or on the surface of the bone, and their histologic grade.