Positron-emission tomography (PET) is an excellent technique, utilizing positron-labeled radiopharmaceuticals which provide a quantitative image of tissue metabolism in the living body. Cancer tissue is characterized by increased forms of metabolism, such as glycolysis and protein synthesis. Consequently, PET makes it possible to evaluate the metabolic activity of cancer tissue. Such information is useful not only for a better understanding of human cancer biology but for utilization in cancer clinics. The potential use of the PET technique for clinical oncology are cancer detection, cancer grading, biological characterization of tumors, and evaluation of therapeutic effectiveness. From this viewpoint, much clinical research has been done and good results obtained in both the characterization of hepatoma and lung cancer, and the grading of brain tumor, showing good correlation with prognosis after cancer therapy. However, PET is not commonly available and has not yet been established as a diagnostic technique in cancer clinics. Further extensive studies will thus be necessary in order to achieve these aspects.