Osteosarcoma, the most frequent bone cancer of children and adolescents, will almost always result in death due to pulmonary metastatic disease unless treated by surgery and effective multidrug chemotherapy. Imaging of the primary tumor is by X-ray and magnetic resonance imaging. Imaging of the chest is by computed tomography, but many questions regarding the interpretation of small, nonspecific findings and how to deal with them remain. The diagnosis must be confirmed by a well-placed biopsy. Chemotherapy is usually initiated prior to definitive surgery. Treatment generally includes high-dose methotrexate, doxorubicin, and cisplatin, with some regimens also incorporating ifosfamide. While limb-saving resections have become standard after completion of skeletal growth, reconstruction in the growing child poses much greater challenges. The use of modern, expandable endoprostheses, but also rotation-plasties and even amputation may all be valid options in selected cases. Histologic response of the primary to preoperative chemotherapy has been identified as an important prognostic factor. Various imaging methods can help to predict tumor response to preoperative chemotherapy, yet all have their limitations. Results of a randomized trial assessing if modifying postoperative treatment based on the extent of response will improve results, EURAMOS-1, are pending. The debate about whether biologic agents or targeted therapies added to chemotherapy might improve outcomes is ongoing. Follow-up to detect late-effects of treatment and recurrences of osteosarcoma must be continued for several decades.