During a medical imaging exam, patient motion such as breathing, cardiac motion, and random adjustments of the head or body can affect the accuracy of the acquired images. Positron emission tomography (PET) is a powerful medical imaging technology that can detect minute amounts of an injected probe targeting specific tissue processes and diseases. Magnetic resonance imaging (MRI) can acquire anatomical and functional images of the body, including information about patient motion. Using a hybrid positron emission tomography and magnetic resonance imaging (PET/MRI) system, we are developing better ways to account for motion and attenuation during PET imaging using MRI images collected at the same time as the PET images are collected. This work will establish the value of our partner organization's existing motion correction methods for PET/MRI in various diseases and develop new PET/MRI methods that correct for cardiac and respiratory motion.