Theory of radiative transfer describes the transmission of the electromagnetic radiation through a medium. The electromagnetic radiation can be emitted, absorbed, scattered by constituents of the medium depending on the composition of the medium and the physical state of its constituents, as well as the wavelength of the radiation itself. Retrieving geophysical parameters from radiation measurements requires to know this kind of interaction which is described through the Equation of Radiative Transfer. In the field of Earth Observations from space, the considered medium is normally the Earth's atmosphere through which the e.m. radiation travel before reaching aerial multi-spectral sensors. Radiative transfer models allow to foreseen spectral radiances at whatever altitude in atmosphere (radiance at the sensor) starting from the knowledge of atmospheric vertical profiles of temperature and chemical constituents concentrations (direct problem). The possibility to retrieve atmospheric temperature profiles and chemical constituents concentrations from multi/iper spectral radiances measurements in selected bands (inverse problem) is the scope of the inversion techniques widely applied in meteorology and of a specific set of sensors devoted to the vertical sounding of the atmosphere. Clouds and scattering particles, like aerosols - requiring the inclusion of additional information on the atmospheric constituents (e.g water phases involved, dimensions and geometry of scattering particles, etc.) - make radiative transfer model more complex.