Osteoarthritis (OA) is a leading cause of disability, resulting in reduced quality of life, at tremendous societal cost. New hybrid PET/MR systems allow for simultaneous, sensitive, and quantitative assessments of early bone activity in OA with PET, which can be correlated with high-resolution quantitative MR methods of soft tissues to study the pathogenesis of OA. We demonstrate promising initial results of simultaneous PET/MR hybrid imaging of knee OA. Results suggest that PET/MR may detect metabolic abnormalities in subchondral bone, which appear normal on MRI. These advancements will allow us to detect and track early and reversible changes in OA.