We consider a new type of nonradiative transitions in nonmetallic solids. The transitions are not accompanied by heat release but by large (compared with interatomic distances) displacements of individual atoms. These instabilities (electrostatic, vibronic, structural), which lead to formation of defects in solid crystals and glasses, are classified. We describe defect-formation processes both in ionic crystals upon decay of self-trapping excitons, and in semiconductors following multiple ionization of atoms near pre-existing charged impurity centers. The mechanisms whereby complex defects are restructured in semiconductors when nonequilibrium current carriers are introduced and when electrons and holes recombine are discussed. The role of current carriers in thermal production of defects is considered. The mechanism of formation of peculiar defects in glassy semiconductors is discussed.