The combination of magnetic resonance imaging (MRI) with positron emission tomography (PET) is expected to result in multi-parametric imaging allowing totally new insights in many diseases. Because classical PET detectors use photomultipliers as readout electronics of the scintillation crystals, they cannot be operated in the environment of the MRI scanner. Therefore, beginning at the level of preclinical applications, different experimental developments towards PET-compatible technology have been addressed by the community. The replacement of the magneto-sensitive photomultipliers by solid-state components, such as avalanche photodiodes and later silicon photomultipliers, represent an initial breakthrough. After a first industrial prototype for brain MR-PET imaging, all major medical imaging manufacturers developed whole-body MR-PET scanners for sequential or simultaneous imaging using the two modalities. This chapter presents an overview of hybrid MR-PET, starting with the first experiments towards MR-PET and arriving at the state-of-the-art equipment in use today.