Commercial combined PET/MRI systems are now available, some of which are capable of simultaneous dual‐modality data acquisition. In terms of PET quantification, MR‐based attenuation correction is a potential drawback, although good progress is being made on solutions for this, especially for brain imaging. In other regards, MR information from combined PET/MRI systems can enhance PET, especially outside the brain, through improved image registration, region‐of‐interest delineation, motion correction, resolution recovery, denoising and additional pharmacokinetic data. This chapter discusses combined PET/MRI designs, the key areas in PET quantification that are affected and clinical/pre‐clinical applications of combined PET/MRI.