[en] Positron emission tomography (PET) is a unique diagnostic method to visualize the distribution of radiopharmaceuticals in vivo and supports detection of metabolic malfunction prior to tissue morphologic disorder. A positron emitted from the radionuclide interacts with an electron and creates two photons with high energy. The location of the radionuclide is identified by coincident detection of these photons penetrating the body. This paper provides brief explanation of characteristics of PET which becomes one of the important tools for molecular imaging research, and then overviews research trends and assignment of depth-of-interaction (DOI)-PET technology. (author)