Osteosarcoma arising on the periosteal aspect of bone comprises a biologically heterogeneous group of neoplasms. The group as a whole may be referred to by a single descriptive term that emphasizes their common site of origin and underscores their malignant osteogenic potential: surface osteosarcoma. Its biologic heterogeneity may be approached via a number of avenues. Detailed description of individual tumors and grading are frequently employed. However, implementation of a classification system based upon reproducible clinical, roentgenographic, macroscopic, and histologic parameters is advantageous. The suggested classification system serves to clearly define parosteal and periosteal osteosarcoma, as well as recognize unusual variants. Most important, it defines therapeutic strategy. The classification system identifies low-grade, biologically indolent forms (i.e., parosteal osteosarcoma and periosteal osteosarcoma) that are best treated by surgery alone. At the same time, it recognizes high-grade forms with significant potential for life-threatening behavior (i.e., 'dedifferentiated' parosteal osteosarcoma and high-grade surface osteosarcoma) that are best managed by multimodality therapy incorporating chemotherapy and surgery.