The future energy system in Europe needs to be decarbonized and thus be based almost exclusively on renewable energy sources. Therefore it is challenged by the intermittent nature of renewables and requires several flexibility options. The interaction between different options and the impact on environment and society are in the focus of this contribution. It is the core objective of this book to analyze and evaluate the development toward a low-carbon energy system with focus on flexibility options in the EU to support the implementation of the Strategy Energy Technology Plan. The analyses are based on a bottom-up modeling environment that considers current and future energy technologies, policy measures and their impact on environment and society while considering technological learning of low-carbon and flexibility technologies.