Power to Gas (PtG) is the conversion of electrical energy (electricity) into chemical energy (gas) by water electrolysis. The hydrogen gas produced can be used energetically (as fuel) or also chemically (in industry). The accompanying diagram shows how this approach functions and the coupling of the different energy sectors. Based on the unavoidable conversion losses in the process chain, the Power to Gas concept is coupled with renewable energy sources in particular. Hydrogen, an ideal companion for renewables, has the potential to store large amounts of energy over long periods of time in chemical form thus levelizing the renewable energy supply in the energy system. In principle, hydrogen can also be reconverted back into electricity; however economically attractive options emerge with the intersectoral linking of zero-emission mobility and the chemical industry. New possibilities for reducing carbon dioxide emissions in these sectors open up.