Multimodality imaging, specifically PET/CT, brought a new perspective into the fields of clinical and preclinical imaging. Clinical cases have shown, that the combination of anatomical structures, revealed from CT, and the functional information from PET into one image, with high fusion accuracy, provides an advanced diagnostic tool and research platform. Although PET/CT is already an established clinical tool it still bears some limitations. A major drawback is that CT provides only limited soft tissue contrast and exposes the patient or animal, being studied, to a significant radiation dose. Since PET and CT scanner are hard-wired back to back and share a common patient bed, PET/CT does not allow simultaneous data acquisition. This temporal mismatch causes image artefacts by patient movement between the two scans or by respiration motion. To overcome these limitations, recent research concentrates on the combination of PET and MRI into one single machine. The goal of this development is to integrate the PET detectors into the MRI scanner which would allow simultaneous data acquisition, resulting in combined functional and morphological images with an excellent soft tissue contrast, very good spatial resolution of the anatomy and very accurate temporal and spatial image fusion. Additionally, since MRI provides also functional information such as blood oxygenation level dependant (BOLD) imaging or spectroscopy, PET/MRI could even provide multi-functional information of physiological processes in vivo. First experiments with PET/MRI prototypes showed very promising results, indicating its great potential for clinical and preclinical imaging.