Response of high-grade osteosarcoma and Ewing sarcoma to preoperative chemotherapy can significantly affect the surgical approach to patients with these tumors and their disease-free survival. This article presents the findings and limitations of different imaging modalities (radiography, angiography, sonography, scintigraphy, and magnetic resonance [MR] imaging and spectroscopy) for evaluating this chemotherapeutic response. Particular emphasis is given to the currently most effective modality--dynamic contrast-enhanced MR imaging.