Patients will undergo DCE-MRI studies prior to chemotherapy, 18-24 days after starting treatment, continue treatment and undergo a third MRI within 25 days prior to surgery. All patients with OS or Ewing sarcoma who will undergo pre-operative chemotherapy are eligible for study. Anti-neoplastic therapy will be determined by the primary physician and will not be impacted upon by this study, although the data will be made available to the physicians involved in the patient's care. The goals of this study are: 1) To determine if pre-operative dynamic contrast enhanced magnetic resonance imaging (DCE-MRI) can reliably predict percent necrosis at definitive surgery in bone tumors after induction chemotherapy. 2) Evaluate the hypothesis that in patients with Ewing or osteogenic sarcoma (OS), the a priori or early (18-24 days into treatment) DCE-MRI study predicts pathological percent necrosis measured at surgery, and disease free survival. 3) Determine if the DCE-MRI results are independent markers of tumor response compared to current clinical markers (LDH, alkaline phosphatase (osteogenic sarcoma only), primary disease site, primary disease size (length and volume), stage). 4) Compare the DCE-MRI results with several key molecular parameters including p53 mutations and CDKN2A deletion in Ewing's sarcoma and RB alterations, CDKN2A deletion, and expression of HER-2, platelet derived growth factor, reduced folate carrier, and pglycoprotein in osteogenic sarcoma. Ewing's sarcoma and osteogenic sarcoma samples will also be submitted for Affymetrix expression microarray analysis. Summary: This project will determine the potential of DCE-MRI to predict tumor necrosis and as an a priori or early marker of tumor response to neoadjuvant therapy