MRI is used in clinical routine for diagnosing brain tumors, but has limitations in identifying tumor grade, true tumor extension and differentiate viable tumor tissue from treatment induced changes and recurrences. Amino acid PET has demonstrated a great potential for defining true tumor volume, differentiate viable tumor tissue from postoperative changes or radiation necrosis, selection of biopsy site, non-invasive grading of gliomas and for treatment planning and therapy response assessment. By combining PET with MRI, the diagnostic accuracy can improve significantly for these patients. More research is however needed to compare the most promising amino acid PET tracers in patients with glioma, but also to assess the diagnostic value of amino acid PET in patients with different brain metastases, where the knowledge concerning the uptake characteristics is limited. Three of the most promising amino acid tracers (\[11C\]-methyl-methionine (11C-MET), \[18F\] fluoro-ethyl-tyrosin (18F-FET) and anti-1-amino-3-\[18F\]fluorocyclobutane-1-carboxylic acid (18F-FACBC)) will be evaluated in 3 substudies in this project; WP1 Aminoacid PET/MRI in low and high grade glioma; WP2 Role of 11C-MET in high-grade glioma Gamma Knife® radiosurgery; and WP3 Amino acid PET in brain metastasis. The main aim of the study is to improve diagnostic accuracy, histopathological tissue sampling, delineation of tumor extent and therapy response assessment in gliomas and brain metastases with amino acid PET/MRI.