AbstractAqueous electrolytes were once the rule for the battery industry. Until the advent of lithium ion batteries, a majority of commercially relevant batteries utilized water as the solvent for ion exchange. The development of the intercalation‐based lithium ion battery upended the industrial aqueous electrolyte paradigm: the high energy density of the lithium‐ion battery was revolutionary but required the use of organic electrolytes capable of passivating strongly redox active electrodes. With the safety of organic electrolytes becoming an issue in the early 1990s, a small community re‐examined aqueous electrolytes for lithium ion batteries. The first such audacious attempt was by Dahn et al., who conceptualized an aqueous lithium‐ion battery chemistry based on electrode materials suitable for the narrow electrochemical stability window of water, sacrificing energy density and cycle life for safety and low cost. The concept of an aqueous lithium‐ion battery was revived in the mid‐2010s with “highly concentrated” electrolytes, expanding the electrochemical stability window of water to regions comparable with nonaqueous electrolytes. Since then, significant efforts have been made around the world, aiming to understand the nature of the interfacial stability in those high‐concentration electrolytes as well as to further make the system viable for practical batteries. This review summarizes these efforts in this emerging frontier of new battery chemistries.