Optical Remote Sensing deals with those part of electromagnetic spectrum characterized by the wavelengths from the visible (0.4 micrometer) to the near infrared (NIR) up to thermal infrared (TIR, 15 micrometer). It regards the collection and interpretation of the e.m. radiation emitted, reflected, adsorbed and transmitted by the observed targets in order to derive their physical-chemical properties and related information. Such a possibility derives from the basic principle of (multi-spectral) remote sensing that is widely supported both theoretically (e.g. atomic and molecular spectroscopy) and experimentally (e.g. spectral signatures catalogues). It states that, in principle (e.g. disposing of sensors with ideal spectral capabilities) the matter-radiation interaction depends on the wavelength of the involved radiation and on specific (e.g. chemical/physical) properties of the matter that can be derived by the spectral analysis of the emerging (emitted, reflected, adsorbed or transmitted) radiation. As far as Earth Observation is concerned, specific related concepts have to be addressed like: the spectral matter-radiation interactions (spectral signature concept), natural sources (e.g. Earth, Sun) of optical e.m. radiation, theory of the Black Body, atmospheric physics and radiative transfer equations in the VIS-NIR and TIR spectral ranges, basic physics of e.m. optical sensors and image systems, physical fundaments of the interpretation of optical radiances collected by multi-hyperspectral passive techniques.