When the surface of a metal crystal is bombarded by inert gas ions, metal atoms are knocked off the surface in certain preferred directions. Evidence is presented to show that some atoms are also knocked into the crystal. A thin disoriented surface layer appears which, if the crystal is an alloy, is of uniform but different composition from that of the original crystal, The importance of these findings to some current ideas about damage due to bombardment is briefly discussed and it is shown that present theories do not account satisfactorily for the observations.