At micro/nanoscale, the general principles of the thermal radiation are failed to solve or explain the majority of radiative problems or phenomena. This chapter will first point out the limitations and reasons of the general principles of thermal radiation by introducing a typical example. Then, some basic concepts, including the role of energy carriers such as photons, electrons, and phonon will be introduced first, followed by the brief introduction of the corresponding governing equations and the influence mechanism in radiative properties. Next, we would like to give a fundamental framework and chart review from macro- to nanoscalethermal radiation, aiming to make the relation and difference between macro- and nanothermal radiation more distinct. Finally, the development of micro/nanoscale thermal radiation in the last decades will be summarized as well.