This book focuses on a subject that is little dis- cussed on its own termsdthe electrification of energy systems and societies. Such a focus dif- fers from most studies examining the electric en- ergy system as such or its future development. Indeed, studies abound in various engineering disciplines on the evolution of electric energy systems. Often these studies also mention how electricity is becoming the main energy carrier, notably replacing the combustion of fossil fuels for power and heat. A burgeoning field of studies also concentrates on how electric energy systems are developing toward “smart grid” type “systems of systems,” where the manage- ment of electricity flows becomes central not only for the production, distribution, and con- sumption of power but also for energy use in transport, buildings, and industry. Such studies span perspectives from several engineering, in- formation, and communication sciences with research on computer science, artificial intelli- gence, software, as well as studies on innovation and technology development, for example. The technical insights of such studies will naturally be surveyed in this book but chiefly to help us better understand electrification as a megatrend that transforms energy systems and societies.