A series of 102 benign osteoblastic tumors of multiple skeletal sites was reviewed, and on the basis of their clinical, radiologic, and pathologic features they were classified into three diagnostic categories: osteoid osteoma, osteoblastoma, and aggressive osteoblastoma. A historic review of the development of the nomenclature of benign osteoblastic tumors, with special emphasis on the evolving concept of aggressive and malignant behavior, is presented. Histologic criteria for the recognition of aggressive osteoblastoma are presented and illustrated in connection with the 15 cases so classified in the present series. The differential diagnosis of aggressive osteoblastoma and low-grade osteosarcoma is discussed. In defining the problem of differentiating locally aggressive osteoblastic lesions from potentially metastasizing tumors, the authors propose that four categories of these osteoblastic tumors can be defined: (1) Innocuous-appearing low-grade osteosarcomas that resemble osteoblastomas histologically. This mimicry accounts for most errors in diagnosis. (2) Rare osteoblastomas that have undergone spontaneous transformation into osteosarcomas. (3) Very rare, clinically and radiologically typical osteoblastomas that show pseudosarcomatous histologic features but pursue a benign course. (4) Locally aggressive osteoblastomas that are likely to recur, do not metastasize, and show characteristic and recognizable histologic features.