An impact−ionization model predicts a negative resistance type of dielectric instability and breakdown in wide bandgap insulators. The instability develops by the injection of electrons from the cathode, impact ionization of the lattice, and a distortion of the electric field which leads to a further increase in impact ionization. For the case of thin films, it is necessary to invoke a nonlocal ionization rate in order to obtain a negative resistance instability. O’Dwyer’s avalanche theory of breakdown, which applies well to semiconductors, cannot be used to predict a negative resistance in the small multiplication limit appropriate to wide bandgap insulators. The impact−ionization model presented here depends upon two critical parameters: the ionization bandgap of the material and the electron−LO phonon scattering length. Evaluated for the exemplary case of SiO2, the model predicts a current density and an average field at breakdown which increase rapidly for film thickness below 200 Å.