AbstractSolid ionic conductors, also called solid electrolytes, transport electric current by means of ions. The best known examples of these usually crystalline compounds include doped ZrO2, as well as AgI, β‐Al2O3, and CaF2. Ionic conduction in solid electrolytes reaches its maximum value if a partial lattice of a solid compound undergoes transition at elevated temperature to a quasimolten state. The ionic conductivity in such solid compounds is then as high as in molten salts. Solid electrolytes have found many scientific and technological applications; thus, they can be used to study thermodynamic and kinetic problems, and to build fuel cells, batteries, sensors, and chemotronic componen