Positron emission tomography (PET) is a radioisotope imaging method for obtaining information on the status of organs and metabolisms within the human body. The imaging principle used in PET is the coincidence detection of two 511 KeV annihilation photons-originated electron-positron annihilation. In practice, a radiopharmaceutical (e.g., F-18-FDG) obtained by uniting a positron emitter radioisotope (e.g., C-11, N-13, O-15, F-18) and glucose molecule is given to the patient by way of blood injection. PET detects these 511 KeV annihilation photons accumulated in the abnormal structures and then three-dimensional imaging of the body is created by the computer. In this review of the imaging principle of PET, scintillation crystals, PET radioisotopes and PET for radiotherapy treatment planning are discussed.