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Destructive breakdown has been studied in Al-SiO-Al capacitors using transmission electron microscopy and electron diffraction, while the capacitors were subjected to controlled electrical stresses. Extensive electrical measurements were made outside the electron microscope on other capacitors. The purpose is to describe destructive breakdown and to relate it to prebreakdown conduction, conditions of fabrication, and capacitor structure. It is found that breakdowns originate at inhomogeneities in the dielectric which appear in electron micrographs as irregularly shaped dark spots about 0.5 μ in diameter. The structure of these dark spots has not been determined. Breakdowns are not concentrated at pinholes, dust spots, or fissures. Breakdown is accompanied by the growth of crystalline silicon. Electrical measurements with a given capacitor reveal two well-defined polarity-sensitive threshold voltages, one for the onset of breakdown and one for the cessation of breakdown. Both are virtually independent of temperature from 80° to 380°K. The voltage threshold for the onset of breakdown varies approximately as w1/2, where w is the dielectric thickness, while the voltage threshold for the cessation of breakdown is found to be independent of dielectric thickness. The diameter of a typical breakdown is 10 to 100 μ. The conductivity of the breakdown site at the onset of breakdown changes by a factor of about 1010 in a time less than 0.1 μsec. Duration of a breakdown is usually about 1 μsec. A breakdown mechanism is proposed that is based upon an electrochemical solid reaction in the presence of the breakdown electric field: Si−O−Si+(breakdown electric field)→Si−Si++O+e,where the term Si−Si+ means the two silicon atoms are in crystalline form and one is singly ionized. A description of the breakdown process is given using this reaction.