After an introduction into the concept of thermochemical energy storage (TCES) via reversible solid – gas reactions special emphasis is given to the class of transition metal salts as promising thermochemical energy storage materials (TCMs). A comparison of gravimetric and volumetric energy storage capacities as well as intrinsic problems of compound volatility at elevated temperatures is used to choose transition metal salts with realistic potential for heat storage and release applications. Furthermore, problems and pitfalls of cyclic applications of the chosen materials in different test rig setups is discussed and the origin of the problems is described. So far, several methods to circumvent these problems, which have been extensively reviewed in general about the performance of TCES under realistic application conditions are under investigation. The lecture will be closed by a strategic outlook and further possible developments for future applications.