By 2020, some European Union energy and climate policies will expire and discussion of a post-2020 policy needs to be initiated. One of the fundamental issues is related to the portfolio of energy sources which can provide the most environmentally-safe, cost-effective roadmap. In this chapter we discuss the challenges of electric energy systems of the future with regard to the integration of sustainable energy resources and smart grids developments. In our view, such a system requires innovation in three related fields: development of sustainable energy sources; development of smart grid technologies for generation of renewable and intermittent sources and storage devices, transmission and distribution controls, and the development of new models to understand the complexity of these types of systems. We discuss the state of the art and the future expectations for these related fields.