The subject of Commission 21 is of a heterogeneous nature and the borderlines to other Commissions are not very well defined. The light of the night-sky comprises a variety of components which are due to various kinds of physical processes taking place at very different distances from the Earth.

In the present report less emphasis is placed on airglow problems as in previous ones in accordance with a recommendation of Commission 21 of 1970. During recent years airglow research became an important subject of geophysics and the new tools offered by rockets and satellites have enormously expanded the observational side. This report tries to concentrate upon aspects of astronomical interest. The review papers on airglow and connected atmospheric problems mentioned in the beginning of Chapter II on the other hand contain valuable information on progress in all sections of this wide field.