Radiative transfer,i.e., the transport of radiant energy through a medium, can be described in several alternative ways, either atmacroscopic or microscopic level. In order to set a common physical background for the applications of radiative transfer to stellar and planetary atmospheres, presented in the second part of this book, a macroscopic representation of the radiation field derived from radiometry, a microscopic picture based on the kinetics of photons and the transport of radiant energy in terms of Maxwell's electromagnetic theory are discussed.