This open access book analyzes the transition toward a lowcarbon energy system in Europe under the aspects of flexibility and technological progress. By covering the main energy sectors including the industry residential tertiary and transport sector as well as the heating and electricity sector the analysis assesses flexibility requirements in a crosssectoral energy system with high shares of renewable energies. The contributing authors all European energy experts apply models and tools from various research fields including technoeconomic learning fundamental energy system modeling and environmental and social life cycle as well as health impact assessment to develop an innovative and comprehensive energy models system EMS.