This paper reviews research in the erosion of fused silica and glass in an effort to gain a better understanding of the ionic polishing process as applied to optical materials. Erosion rates depend on ion mass, ion energy, target temperature, angle of incidence, and target material, and can also depend on vacuum pressure. The paper also considers other effects that accompany ionic bombardment of insulators, such as nature of the eroded surface, contaminate films, surface layer alterations, secondary electron emission, and gas trapping and release.