  The purpose is to introduce in a clear and direct way the students of
undergraduate courses in physics and/or astronomy to the subject of radiative
transfer. A pedagogical revision is made in order to obtain the radiative
transfer equation, its restrictions and the different types of interactions
present between the radiation and the matter. Because in the classical
literature about radiative transfer the covariance is not fully developed, we
show in an explicit manner detail calculations and then we discuss the
relativistic effects.
