Positron emission tomography (PET) is a nuclear medicine imaging method with increasing relevance for the diagnosis, prognostication, and monitoring of glioblastomas. PET provides additional insight beyond magnetic resonance imaging (MRI) into the biology of gliomas, which can be used for noninvasive grading, differential diagnosis, delineation of tumor extent, planning of surgery, and radiotherapy and post-treatment monitoring. In clinical practice, two classes of radiotracers have been used predominantly for imaging purposes, namely glucose metabolism tracers and amino acid transport tracers. Both classes of tracers can provide information on grading and prognosis of gliomas, but amino acid tracers, which exhibit lower uptake in normal brain tissue, are better suited for delineation of tumor extent, treatment planning, or follow-up than 18 F-2-fluoro-2-deoxy-D-glucose (18 F-FDG). Owing to the progress in PET imaging using radiolabeled amino acids in recent years, the Response Assessment in Neuro-Oncology (RANO) working group, an international effort to develop new standardized response criteria for clinical trials in brain tumors, has recently recommended amino acid PET as an additional tool in the diagnostic assessment of brain tumors.