ConspectusThe recent discovery of "water-in-salt" electrolytes has spurred a rebirth of research on aqueous batteries. Most of the attention has been focused on the formulation of salts enabling the electrochemical window to be expanded as much as possible, well beyond the 1.23 V allowed by thermodynamics in water. This approach has led to critical successes, with devices operating at voltages of up to 4 V. These efforts were accompanied by fundamental studies aiming at understanding water speciation and its link with the bulk and interfacial properties of water-in-salt electrolytes. This speciation was found to differ markedly from that in conventional aqueous solutions since most water molecules are involved in the solvation of the cationic species (in general Li+) and thus cannot form their usual hydrogen-bonding network. Instead, it is the anions that tend to self-aggregate in nanodomains and dictate the interfacial and transport properties of the electrolyte. This particular speciation drastically alters the presence and reactivity of the water molecules at electrified interfaces, which enlarges the electrochemical windows of these aqueous electrolytes.Thanks to this fundamental understanding, a second very active lead was recently followed, which consists of using a scarce amount of water in nonaqueous electrolytes in order to control the interfacial properties. Following this path, it was proposed to use an organic solvent such as acetonitrile as a confinement matrix for water. Tuning the salt/water ratio in such systems leads to a whole family of systems that can be used to determine the reactivity of water and control the potential at which the hydrogen evolution reaction occurs. Put together, all of these efforts allow a shift of our view of the water molecule from a passive solvent to a reactant involved in many distinct fields ranging from electrochemical energy storage to (electro)catalysis.Combining spectroscopic and electrochemical techniques with molecular dynamics simulations, we have observed very interesting chemical phenomena such as immiscibility between two aqueous phases, specific adsorption properties of water molecules that strongly affect their reactivity, and complex diffusive mechanisms due to the formation of anionic and aqueous nanodomains.