In patients less than 30 years old, osteosarcoma is the most common primary malignant bone tumor. However, there is a bimodal age distribution due to secondary osteosarcoma in patients with Paget’s disease and prior radiation therapy. The most common type, accounting for approximately 75% of cases, is the high-grade intramedullary variant (also known as central or conventional osteosarcoma). 75% occur in patients in the second and third decades of life (15-25 years old). Though classically occurring at the end of long bones, location is not a useful discriminator, as they may occur anywhere in the skeleton. They are destructive lesions, and though usually demonstrating bony sclerosis, occasionally are entirely lytic. There is disruption the cortex, but usually no bony expansion. There is typically an aggressive periosteal reaction (Codman's triangle, laminated, hair-on-end, or sunburst patterns), and soft-tissue masses are present in the majority of patients (80%-90%). Tumors are often large at the time of presentation (>6 cm) due to the quick doubling time (20-30 days). The 5-year survival rate is 60%-80%. Pulmonary metastases (CXR or chest CT) are associated with a poor prognosis. Evaluation for metastatic disease to bone includes an MDP whole body bone scan, which demonstrates uptake on all three phases. Thallium imaging is useful to assess tumor activity, particularly in follow-up after chemotherapy. MR evaluation is valuable look for skip lesions in the same bone and in surgical planning in order to precisely define the extent of marrow, soft tissue and joint space involvement.