Neoadjuvant chemotherapy (NAC) for osteosarcoma (OS) and Ewing sarcoma (ES) is associated with significant immediate and long-term complications, particularly difficult to endure in adolescent patients. Tumor response is assessed only at resection, often after the patient has received months of potentially toxic and ineffective therapy. Surgical approaches in this setting are extensive and life changing, with amputations not uncommon. Poor response to NAC is the single most important prognostic indicator in localized OS/ES. Early identification of those patients unlikely to benefit from the prescribed regimen could have significant clinical implications and allow for earlier adjustments in the patient's therapy. In patients with OS/ES there remains a compelling yet unmet need for more advanced quantitative, noninvasive imaging methods that can be deployed early after the initiation of treatment and which are capable of longitudinally measuring quantitative changes in relevant physiological, metabolic and/or biophysical parameters that can serve as reliable surrogates, or even predictors, of long-term tumor response to intervention, including pathological response at surgery. In this pilot study we will use multi-parametric 3 Tesla (3T) MRI, deployed before and after the first cycle of NAC, to correlate early changes in imaging biomarkers with the patient's eventual histopathological response at surgical resection. We will measure treatment-induced changes in: 1) protein content, measured via the amide proton transfer asymmetry (APTasym) using chemical exchange saturation transfer (CEST) MRI); 2) tumor fibrosis, measured via the magnetization transfer ratio (MTR) using magnetization transfer (MT) MRI); 3) tumor cellularity, measured via the apparent diffusion coefficient (ADC) using diffusion-weighted MRI); and 4) tumor perfusion, measured via the volume transfer coefficient (Ktrans) using dynamic contrast-enhanced DCE-MRI. The relevance and future clinical impact of each of these imaging biomarkers (alone or in combination) in OS/ES is potentially very high.