The knowledge of disease mechanisms and the identification of markers of disease development and progression, and for therapy evaluation, are rapidly expanding.Early diagnosis, possible disease prevention and individual therapy stratification are important goals in health care.Nuclear medicine and radiology are potentially able to satisfy these increasing demands for diagnosis, prevention, pathophysiological understanding and treatment possibilities.An increasing number of biomarkers, drugs, antibodies and peptides, contrast agents and MRI sequences have also been developed for these purposes.Together with this continuing search for the "holy grail" radiopharmaceutical, the use of hybrid imaging is another important development in the field.A recent development in hybrid cameras is the introduction of combined PET and MRI.Despite the intriguing novelty, it is important to evaluate carefully, in a cost-benefit manner, the possible advantages and clinical applications of such an expensive tool. Advantages in general of hybrid PET/MRI systemsWhile PET enables the acquisition of functional data at the molecular level, MRI provides superior soft-tissue resolution and anatomy along with semiquantitative macromolecular information.Hybrid PET/MRI systems can combine these imaging characteristics.PET/MRI systems can be either simultaneous or sequential.Although imaging data from separate PET and MRI systems may be combined, simultaneous imaging systems have major theoretical advantages that could be of interest to the whole medical community.The general advantages of a simultaneous PET/MRI system as compared to conventional PET/CT systems are:Recording of dynamic and moving phenomena, which could enable tracers with short half-lives to be studied & Identical position of the patient during image acquisition with both modalities leading to a substantial reduction in motion artefacts due to heart beating, intestinal motion and breathing.& Absolute match between the tissue information of both modalities under the same physiological conditions.& Contrast-enhanced MRI information (e.g. on perfusion and blood flow) can be used in the pharmacokinetic modelling of the PET data, resulting in an improvement in PET reconstruction and data analysis.& Better localization of the PET signal within the soft tissues.& No radiation burden from the MRI part.& Better application of the one-stop-shop principle for simultaneous diagnostic-quality acquisition of nuclear medicine and radiological images.& More structural, functional and molecular imaging at the same time.On the other hand, compared to PET/CT, PET/MRI examinations may result in a lower compliance by the patient,