E.M. Radiation can be absorbed, scattered, emitted and transmitted by the matter. The results of such interactions (i.e. the fraction of incident radiation that is absorbed, scattered or transmitted) or emission process (i.e. the fraction of actually emitted radiation in comparison with the one expected from a black-body at the same temerature) strongly depend on the radiation wavelength and on specific chemical (e.g. composing atoms and molecules as well as their arrangement within solid cristals) and physical (e.g. Temperature, Dimensions and Shape, Roughness) properties of the matter. In some case, the result of Radiation - Matter interaction is strongly affected by observational conditions. For instance, over some angular distance between the directions of incidence and the one of measurement of the radiation, sun-glint can occur which completely mask any other results. A basic principle of the remote sensing put univocally in relation spectral absorbance, reflectance, transmittance and emissivity, curves achievable by multi-spectral EO measurements, with matter having specific chemical/physical properties. Theoretical models of radiation-matter interaction at the Earth's surface and through the atmosphere provide then suitable strategies for retrieving, from multi-spectral measurements of the radiation leaving the Earth, the most relevant chemical/physical properties of the matter composing its surface and atmosphere.