This issue discusses the clinical application of PET Imaging in assessing brain tumors, Including what a neuro-oncologist's expectations should be. One article discusses how PET can help in developing reliable response evaluation criteria in brain tumors; another reviews modern tracers for brain tumors. The evolving role of PET-MRI in brain tumors is examined. Parametric mapping of multiple PET tracers with MRI response evaluation is reviewed. Another article discusses the role of early and delayed PET imaging and novel quantitative techniques in hybrid imaging for brain tumors. The perspective of pediatric imaging is also given.