This article reviews the fundamental experimental and theoretical results on the nature of radiation defects and the mechanism of their production in ionic (mainly alkali-halide) crystals. We show that one can produce high concentrations of radiation defects in these crystals by irradiating with photons and particles whose energies do not suffice to displace atoms from lattice sites in elastic collisions with them. On the other hand, excitons and band holes in these crystals have a tendency to autolocalize in the regular lattice. In certain electronic states, excitons prove to be unstable with respect to decay into Frenkel' defects. This is the reason for the low radiation stability of many ionic crystals. We discuss the mechanisms of the above-cited instability and of secondary processes (localization, recombination, charge transfer) in which the products of exciton decay participate, as well as a possible role of such processes in other types of solids.