I will first review a series of our mechanistic studies of flash sintering [Scripta 146:260 (2018)]. First, we showed that flash sintering generally starts as a thermal runaway and developed a quantitative model [Acta 94:87 (2015)]. In addition to naturally occurred thermal runaway, we also showed that a bulk phase transition or a grain boundary complexion transition can trigger flash via a “forced” thermal runway [Acta 181:544 (2019)]. Second, we showed that ultrahigh heating rates of ~200 K/s enable the ultrafast sintering with and without an applied electric field [Acta 125:465 (2017)]. Subsequently, a general ultrafast high-temperature sintering was developed [Science 368:521 (2020)]. Although we have demonstrated that ultrafast sintering can be achieved without an electric field, we have observed a spectrum of electric field (electrochemically) induced microstructural evolution phenomena [Nature Communications 12:2374 (2021); Adv. Eng. Mater. https://doi.org/10.1002/adem.202201767 (Early View)]. On-going studies and unpublished results will be presented and discussed.