Publisher Summary This chapter provides a brief overview of sodium-sulphur secondary batteries. The sodium-sulphur battery is a relatively high ampere hour rechargeable battery, envisaged as a power source on electric vehicles and trains. Its operation relies on the property of β-alumina, namely that it combines very low electronic conductivity with an unusually high ionic conductivity, which is specific to sodium ions as charge carriers. The cell voltage, 2.08 V, is derived from the chemical reaction between sodium and sulphur to produce sodium polysulphide; and the theoretical energy density is about 750 W h/kg, is high compared with that of the lead-acid battery. The novelty of the properties discovered in sodium r-alumina may be estimated from the fact that, although its melting point is around 2000°C, several amps per square centimeter of current may be passed across the electrolyte at 300°C. The cell operates at about 350°C. The key to the rapid sodium ion mobility lies in the crystal structure and the low potential energy path for sodium migration that this produces.