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The effect of DC electric field on sintering, and on the electrical conductivity of undoped rutile, TiO2 (99.99%), has been investigated at fields ranging from 0 V to 1000 V/cm. The experiments were carried out at a constant heating rate of 10°C/min with the furnace temperatures reaching up to 1150°C. The sintering behavior falls into two regimes: at lower fields, up to 150 V/cm, sintering is enhanced, but densification occurs gradually with time (Type A or FAST sintering). At higher fields sintering occurs abruptly, and is accompanied by a highly nonlinear increase in conductivity, which has been called flash sintering (Type B or FLASH sintering). Arrhenius plots of conductivity yield an activation energy of 1.6 eV in Type A and 0.6 eV in Type B behavior; the first is explained as ionic and the second as electronic conductivity. The evolution of grain size under both types of sintering behavior are reported. These results highlight that the dominant mechanism of field-assisted sintering can change with the field strength and temperature. We are in the very early stages of identifying these mechanisms and mapping them in the field, frequency, and temperature space.