In a world of sustainable energy supply, the latter will be predominantly generated, distributed and consumed in the form of electric power and hydrogen. In order to balance supply and demand, for storage purposes and to meet the specific requirements of the different end users, we need powerful energy transformers in both directions-fuel cells and electrolyzers. Though based upon a simple electrochemical effect, tough demands on a water splitter, with respect to efficiency, operability, safety and costs make it a modern electrolyzer and a piece of engineering excellence. Furthermore, the general aspects of electrolyzer design and performance will be discussed.