Positron emission tomography (PET) and single photon computed emission tomography (SPECT) have been evaluated in several studies for radiation treatment planning in patients with primary brain tumors. PET with the glucose analogue fluorodeoxyglucose has been found to be of limited use for radiation treatment planning because the high physiologic glucose use of normal gray matter makes delineation of tumors challenging. In contrast, there is considerable evidence that PET or SPECT with radiolabeled amino acid or amino acid analogues provides valuable information for the delineation of gliomas. Increased amino acid uptake has been found to be a more specific marker for viable tumor tissue than signal abnormalities on MRI. In addition, increased amino acid uptake is frequently observed in tumor areas that have not caused a disruption of the blood brain barrier. Therefore, PET and SPECT with radiolabeled amino acids provide a unique opportunity to visualize the infiltrative growth of gliomas and use this information for radiation treatment planning.