Flash sintering is a new sintering technique in which an electric field is applied across a ceramic material and current flows directly through the material at high temperature. This process results in a rapid sintering of ceramics in seconds. Two mechanisms are proposed for the cause of the rapid sintering characterized by flash sintering: Joule heating through electronic conduction and an avalanche of defects induced by ionic conduction. In situ studies using Energy Dispersive X-Ray Diffraction (EDXRD) provide us with two critical results in determining the dominance of one or the other mechanism on different ceramics: temperature and inhomogeneity in the lattice expansion. Temperature calibration to conventionally heated specimen provides us with reliable estimates of the specimen temperature during flash to determine whether the sample temperature is too low for sintering to occur through heating alone. Ionic conduction is shown when there is a difference in the lattice expansion along the profile of the sample due to the difference in path that oxygen ions must take to reach the positive electrode. We have studied three materials systems in order to investigate the predominance of each mechanism: ZnO, an electronic conductor, TiO2, a mixed ionic-electronic conductor, and 8YSZ, an ionic conductor.