The directional etching capabilities of certain plasma-assisted etching configurations are usually attributed to energetic positive ion bombardment. In some gas-surface systems, (e.g., Si-F), energetic ion bombardment can cause a dramatic increase in the rate of reaction between gas phase species and solid surfaces. However, in other gas-surface systems (e.g., Al-Cl2), no significant increase in the reaction rate is seen upon initiation of energetic ion bombardment; in fact, sometimes a decrease is observed. In these latter situations, if directional etching is to be obtained, additional condensible species must be present in the gas phase (e.g.,CClx,BCIx) which tend to deposit on surfaces forming thin protective layers. The etch rate of these so-called ``blocking'' layers must be accelerated by energetic ion bombardment. Recent work will be described in which the primary role of ion bombardment in ion-assisted surface chemistry is discussed. That is, which of the steps in an etching process (reaction or chemisorption, product formation, product desorption) is directly influenced by energetic ion bombardment?