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Positron emission tomography / computed tomography (PET/CT) and PET / magnetic resonance (PET/MR) with several tracers has been used to investigate the cellular physiopathology of brain tumors. For prediction of tumor grading, [18F]-fluorodeoxyglucose ([18F]-FDG) is more accurate than most amino acid tracers, while amino acid tracers, typically 11C-methionine, L-3,4-dihydroxyphenylalanine ([18F]-fluorodopa) and to a lesser extent O-(2-[18F]-fluoroethyl)-L-tyrosine, are more accurate than [18F]-FDG for the distinction between tumor recurrence and radiation necrosis. Changes after therapy as well as baseline tracer uptake in untreated patients have prognostic value. Post-surgical scans can be used for identification of residual tumor. PET, in combination with MR, is increasingly used for the definition of the tumor volume that has to be targeted by radiation therapy. Identification of the metabolically active volume can also be used to direct stereotaxic biopsy. Other tracers have been developed to explore different biochemical processes, for example hypoxia ([18F]-fluoromisonidazole), DNA synthesis (3-deoxy-3-[18F]-fluorothymidine, and membrane proliferation (radiolabeled choline). However, these radiopharmaceuticals presently have a more uncertain role in the clinical routine, even though some interesting results are emerging from the first clinical studies. The recent development of PET/MR hybrid scanners has represented a technological breakthrough with immediate consequences on improved diagnostic and research capabilities, as well as on logistical issues favorable for the patient, who can perform two examinations at a single time (“one-stop-shop”). This is particularly true for simultaneous systems in which the PET is within the magnetic field and the traditional PET detection system, classically based on photomultipliers, is replaced by magnetic field-insensitive avalanche photodiodes or silicon-based elements. The combination of the PET/ and MR has already proved to provide a complex synergistic effect. However, the use of PET/MR scanners is currently limited to restricted research centers owing to high costs, demanding maintenance, and necessary interdisciplinary personnel.