Neuroinflammation which involves microglial activation is believed to be closely associated with the progression of neurodegeneration in Alzheimer's disease. The activation of microglia is receiving rising attention as a diagnostic and therapeutic target; however, it is important to distinguish its state and phenotype of the microglia. Positron emission tomography (PET) is a non-invasive method that can help visualize the changes of molecules associated with microglial activation in neurological disease under in vivo conditions. In this review, recent PET preclinical and clinical studies, and new molecular targets for the imaging of microglia are described.