Radiative heat transfer or thermal radiation is the science of transferring energy in the form of electromagnetic waves. Unlike heat conduction, electromagnetic waves do not require a medium for their propagation. Therefore, because of their ability to travel across vacuum, thermal radiation becomes the dominant mode of heat transfer in low pressure (vacuum) and outer space applications. Another distinguishing characteristic between conduction (and convection, if aided by flow) and thermal radiation is their temperature dependence. While conductive and convective fluxes are more or less linearly dependent on temperature differences, radiative heat fluxes tend to be proportional to differences in the fourth power of temperature (or even higher). For this reason, radiation tends to become the dominant mode of heat transfer in high-temperature applications, such as combustion (fires, furnaces, rocket nozzles), nuclear reactions (solar emission, nuclear weapons), and others. This chapter introduces the fundamentals of thermal radiation, and covers the radiative properties of combustion systems, spectral models and global models, radiative transfer equation solution methods, and turbulence–radiation interactions. Together with the material on combustion and turbulence–chemistry interactions in the previous chapter, this provides the foundation that is needed for the examples that are presented and discussed in subsequent chapters.