Positron emission tomography (PET) facilitates the evaluation of molecular aspects and metabolic alterations that are fundamental in detecting of malignancies, characterization of tumor stage and assessment of therapeutical response, and tumor recurrence. The main advantage of PET is its high sensitivity in identifying of areas of cancerous involvement at an early stage. In general, the accelerated radiotracer activity occurs before anatomical structure changes. The main difficulty with PET is the lack of an anatomical reference frame. Magnetic resonance imaging (MRI) is an excellent morphological imaging modality with a high anatomical resolution. Whole-body MRI produce large amounts of image data, resulting in the possibility of overlooking subtle pathological findings. The fusion of PET with MRI can compensate for their disadvantages and therefore offers several advantages in comparison to PET or MRI alone. The combination of these two excellent diagnostic imaging modalities into a single scanner improves the diagnostic accuracy by facilitating the accurate registration of molecular aspects and metabolic alterations of the diseases with exact correlation to anatomical findings and morphological information. Whole-body PET/MRI is a very promising diagnostic modality for oncological imaging and for use in cancer screening in the decades to come due to the considerably lower radiation exposure in contrast to PET/CT and the high soft tissue resolution of MRI.