The problem of phase changes in halides, oxides, carbides, and borides as produced by particle bombardment is considered, with the object of reducing the known examples to the minimum number of mechanisms. The phase changes are grouped topologically, the first group consisting of those which are confined to the outer surface. These are most often caused by electronic sputtering, but near-threshold elastic collisions may be important with substances like Ta2O5 where the components have a large mass difference. The second group consists of changes which extend beyond the outer surface. Such changes can be initiated by an outer-layer change, by bombardment-induced redistribution, and, in principle, by Gibbsian segregation. The third group consists of bulk changes brought about with inert beams and includes the more conventional phase changes. Most examples fit rather well into subgroups involving, respectively, dispersed point defects (e.g. randomization of NbO), mobile point defects (e.g. metal colloid formation in a halide), or the cascade itself (e.g. some instances of amorphization). The final group consists of bulk changes brought about with chemically active beams. As with the third category, it is convenient to subdivide according to whether the point defects are dispersed (e.g. H+ implanted in an oxide) or mobile (e.g. metal ions implanted in an oxide at elevated temperatures). © 1981.