The interface region which lies between the meteorological atmosphere of the Earth and "outer" space is a source of abundant optical radiation. The purpose of this paper is to provide the optical instrumentation engineer with a generalized understanding and a summary reference of naturally-occurring aerospace radiation phenomena. The colors of the radiation extend over the full optical spectrum from ultraviolet through the infrared. The emissions, observed during both day and night times, are rich in Zine and band spectra. The parameterization of atmospheric light by frequency (or photon energy) and by spectral radiance is discussed. The sources of the natural light from the gases of the atmosphere are grouped into four categories: (1) airglow mechanisms, (2) thermal processes, (3) scattering phenomena, and (4) auroral excitations. An overview of the characteristic spectral occurrences and intensities is given.