The mechanisms of reversible and irreversible failures that occur in microwave semiconductor devices, microcircuits, and microprocessors under the action of powerful electromagnetic pulses, either single or periodic, are analyzed. It is shown that, in microprocessors, failures of both types are generated by theelectrothermal instabilities, which develop in negligibly small volumes of a device. The dependences of the threshold energy of failures on the pulse amplitude, length, and repetition rate are explained. The results of the calculation are consistent with the experimental data.