Motion is a frequent problem in magnetic resonance-positron emission tomography (MR-PET) acquisitions, leading to significant degradations of the image quality. This chapter gives an overview of this issue and potential remedies. First, different ways of measuring the intra-scan motion are discussed. This is sub-divided into external device-based PET-based and MR-based motion detection and tracking. Given that MRI-based methods can be relatively fast, they lend themselves to retrospective as well as prospective correction; in retrospective correction the motion information is used to correct flawed k-space data after the scan is completed, i.e. during reconstruction or post-processing, whereas in prospective motion correction the motion information is used to correct the MRI measurement itself in real time while the scan is still running. The goal of prospective correction is to acquire data that are unaffected by any motion that occurs during the measurement. Thereafter, several different motion correction techniques are presented, which are able to counter the negative effects of motion in both MRI and PET.