A review of the basic concepts of Positron Emission Tomography is presented, including modern tracers. The chapter focuses on the populating reactions typically used to get radionuclides, their decay channels, and the physical mechanisms that can be used for detection and therapy. The chapter also analyzes the differences between conventional production (cyclotron) and laser-driven production of such unstable tracers and discusses the situations where lasers can add value to the radioisotope production process.