Molecular imaging is the amalgamation of molecular biology and imaging technology in a unique way that enables in vivo observation of molecular biological processes without altering the process or organism being studied. Positron Emission Tomography (PET) is the most advanced form of molecular imaging suitable for broad application in human. Since positron-emitting radionuclides of elements such as C, N, O, and F can replace the stable analogues in drugs and biomolecules of fundamental biochemical principles, it is possible to synthesize PET probes with the same chemical structure as the parent unlabeled molecules without altering their biological activity. Fundamental biochemical principles comprise several potential targets including the receptors on the tumor surface, targeting agents based on increased metabolic demands of the cancer, and potentially enzymes or processes which are related to cell growth and survival. Characteristics of the microenvironment of tumors, including tumor perfusion and hypoxia, can also be targeted as well as elements of the tumor stroma. In brain tumors, molecular imaging with PET might allow (1) Differential diagnosis and grading, (2) Determination of prognosis, (3) Determination of the exact localization, extent, and metabolic activity of biologically active brain tumors for establishing the target for therapy, (4) Evaluation of the response to treatment, (5) Differentiation between treatment induced lesions and residual or recurrent tumor tissue, (6) Evaluation of function changes within the surrounding brain tissue which need to be assessed for the determination of the pharmacodynamic and neurotoxicity of therapeutic agents. Along with the advances in molecular biology, the effectiveness of PET with noninvasive biomarkers will become increasingly important.