Most of what we know about the cosmos comes from the study of the electromagnetic radiation we receive from it. The matter generates the radiation and this, in turn, is modified when it interacts with the matter. The processes that produces and modifies radiation gives it signals that astronomers interpret to infer the physical and chemical conditions of the universe. This text has been prepared by two recognized experts in this area of worldwide astrophysics and allows to clearly and didactically understand the theory of radiative transfer and its astrophysical applications. The book includes chapters of stellar atmospheres, photometric and spectroscopic observations, traditional problems and computational challenges.