Preclinical, translational, and clinical research projects using both positron emission tomography (PET) and magnetic resonance (MR) imaging at Massachusetts General Hospital are actively supported by the NIH and other federal agencies. These programs are addressing critical issues in oncology, psychiatry, neurology, cardiology, pulmonary, endocrinology, communication disorders, and basic neuroscience and physiology. Whereas most of these imaging studies are carried out as separate PET/CT and MR acquisitions, the recent advent of technologies that permit the construction of hybrid PET/MR scanners now makes it possible to perform both procedures in a single session. Advantages of imaging with PET/MR include reduced radiation dose because non-ionizing MR replaces x-ray CT for anatomical orientation as well as improved workflow and reduced procedure time which are essential for compromised patients such as Alzheimer's disease or stroke patients. Using a PET/MR system that is capable of simultaneous (as opposed to sequential with a shared bed) acquisition of PET and MR data, it is possible for instance to use fast MR imaging to track respiratory or cardiac movement to correct the PET data for resolution-degrading motion artifacts, or to monitor molecular processes in the brain with PET while concurrently tracking cerebral activation with fMRI. MGH main campus does not have a dedicated research whole-body PET scanner since 2013 nor a research MRI, and while human research subjects can be scheduled on clinical MR systems these facilities prohibit animal studies. The proposed PET/MR system will fill the critical need for the main campus NIH funded users who require whole body PET imaging and specifically for NIH funded users where simultaneous PET/MR data can enhance or is essential to their research.