The radiation that propagates in an extended medium is subject to continuous emission and absorption processes that modify its intensity and spectral distribution. These phenomena are governed by the equation of radiative transfer that we have formally obtained in Chap. 11 from the principles of quantum electrodynamics. By solving this equation, it is possible to relate the observed properties of the radiation emerging from an extended medium with the intrinsic properties of the plasma responsible for its emission. In this chapter, we explore this issue by analysing in detail the prototype case for which the theory of radiative transfer was developed, namely the case of stellar atmospheres. The considerations presented here can be easily extended to deal with specific problems encountered in the analysis of the propagation of radiation in non-astrophysical environments, such as laboratory plasmas or the Earth’s atmosphere.