This article reviews possible clinical applications of positron emission tomography (PET) in brain tumor patients. PET allows quantitative assessment of brain tumor pathophysiology and biochemistry. It therefore provides different information about tumors when compared to histological or neuroradiological methods. Common clinical indications for PET comprise tumor delineation and identification of the metabolically most active tumor regions (target for biopsy, differentiation of viable tumor from necrosis). Further, the spatial relation between brain activated e.g., by speech, and the tumor bulk can be explored by activation studies. PET could also aid in the prediction of treatment response by measurement of tumor perfusion or hypoxia. Moreover, PET tracers could identify treatment targets e.g., gene products. The latter topic has not been systematically evaluated in human patients.