The special situation of insulators necessitates a discussion whether the classical collision theory which had mainly been developed with view to metallic systems still holds in ionic or covalently bound solids. Special problems which should be treated are: (a) changes in the interaction potentials due to the localization of electrons of the target atoms, (b) the influence of charge states of the projectiles, which due to slower neutralization processes, might be higher on the average in insulators than in metals, (c) the effect of specific structure, multielemental composition on the trajectories (“focussing”), (d) the importance of covalent binding of the molecular units, and (e) the energy loss via chemical interactions by formation of stable bonds at low kinetic energies and transient ones at higher energies (“chemical stopping”). These problems will be treated by a comparison of experimental results with theory and computer simulation of collision cascades.