Energetic ion bombardment is responsible for the anisotropic etching which can be obtained in reactive ion.etching. This has been recognized for many years but the detailed mechanisms involved in this process are still not well understood. In this paper, the various phenomena resulting from ion bombardment of a surface in a reactive gas glow discharge will be summarized. The discussion will be limited to chemical dry etching environments, that is, the products of the reactive gas-surface interaction are volatile at or slightly above room temperature. The phenomena which will be discussed are chemical sputtering, ion-induced mixing, ions as a source of reactants, physical sputtering, and the effect of prior ion bombardment on selected gas-surface reactions.