IntroductionThe last 200 years have witnessed an incredible increase in energy use worldwide. In recent decades, it has become clear that the way in which this energy is supplied is unsustainable and both short-and long-term energy security are at the top of the political and societal agenda. Scenario studies, which chart possible futures, typically show small incremental changes against a "business-as-usual" (BAU) reference. In contrast, evidence suggests that we should be able to meet our energy demand entirely from renewable sources, given their abundance: Global final energy use was ~310 EJ in 2007 (~500 EJ in primary energy terms) [1], whereas technical potentials for renewable energy sources range in the order of hundreds to thousands of exajoules per year.In an attempt to reconcile these figures, the Ecofys energy scenario provides a comprehensive analysis, examining all energy uses worldwide, all carrier forms (e.g., electricity or fuel), and all purposes (heat in buildings or heat in industry).The key question which guided our study was: "Is a fully sustainable global energy system possible by 2050?"We conclude that an (almost) fully sustainable energy supply is technically and economically feasible, given ambitious, but realistic, growth rates of sustainable energy sources. The path to achieving this system deviates significantly from BAU and sometimes difficult choices must be made on the way. The work presented in this chapter is an adapted version of a larger report published in 2011, and subsequent scientific publications [2][3][4].