Electron heating in silicon dioxide (SiO2) at electric fields ≲5 MV/cm is demonstrated using three different experimental techniques: carrier separation, electroluminescence, and vacuum emission. Gradual heating of the electronic carrier distribution is demonstrated for fields from 5 to 12 MV/cm with the average excess energy of the distribution reaching ≳4 eV with respect to the bottom of the SiO2 conduction band edge. Off-stoichiometric SiO2 (OS-SiO2) layers are shown to behave similarly to very thin SiO2(≲70 Å in thickness) with a transition occurring from ‘‘cool’’ to ‘‘hot’’ electrons as the conduction mechanism changes from direct tunneling between silicon (Si) islands in the SiO2 matrix of the OS-SiO2 material to Fowler-Nordheim emission into the conduction band of the SiO2 regions. The relationship of electron heating to electron trapping, positive charge generation, interface state creation, and dielectric breakdown is treated. The importance of various scattering mechanisms for stabilizing the electronic field-induced heating in the SiO2 and preventing current runaway and impact ionization is discussed. Scattering may be due to disorder, trapped charges, and acoustical phonons, as well as longitudinal optical phonons.