Efficiency effects in a renewable-based energy supply are becoming increasingly important. With the transition to renewable energy sources, new demands for efficiency across the entire energy system arise. This requires considerations across a broad thematic spectrum, from the general requirements for an energy system to the various forms of energy efficiency, as well as the efficient alignment of supply and demand. Fundamental conceptual, physical, and economic aspects are also addressed, enabling readers from diverse professional backgrounds to access the material and providing the opportunity to organize the systematics of the energy transition within their own framework.