We report on the use of high electric fields applied via scanning tunneling microscopy (STM) under ultrahigh vacuum conditions to induce voids and dielectric breakdown in ultrathin gamma'-Al2O3 films (similar to7 Angstrom thick) grown on Ni3Al (111) substrates. Voltage pulses (bias voltages of 0.1-6 V, either polarity) are applied to a specific location on the oxide surface with the feedback current loop operative to prevent tip/sample physical contact. Subsequent STM constant current imaging reveals the effects of high field on surface composition and topography. The results show that dielectric breakdown occurs at a field of 12.3 +/-1.0 MV/cm. Breakdown is marked by the creation of features with apparent size of 8-25 nm high and 100-250 nm wide in the constant current STM images, and by the loss of the insulating nature of the oxide as deduced from current/voltage spectroscopy. Constant height scanning indicates that these features are caused by electronic changes in the oxide and not by mass transport. At lower electric fields, small voids that are 0.2-0.8 nm deep can be created. STM images taken at a 2 vs 0. 1 V gap voltage show that these voids are located at the oxide/metal interface, where they grow into the metal. Repeated application of a lower field within the void region leads to dielectric breakdown, even though the same field does not induce breakdown in regions away from the void. The results provide direct evidence of the ability of localized defects to decrease the barrier to dielectric breakdown in an ordered oxide film.