In times of the German energy transition ("Energiewende") chemical storage technologies become increasingly important. They allow the accumulation of excess electrical energy from wind- and solar power plants in form of hydrogen. Hydrogen itself can be stored in pressure tanks, through liquefaction and physical or chemical adsorption. If needed, the stored hydrogen can be reconverted to electricity via fuel-cells. Alternatively, hydrogen can be converted to methane or diesel. In this contribution, a brief overview of the state of the art in water electrolysis is given, since in the scope of the "Energiewende" it represents a key technology.