A sustainable European energy system, mitigating climate change and solving a number of other key environmental problems, will require massive reliance on renewable energy sources combined with a sharp increase in energy productivity. Considering that most of the technologies necessary for such a development are already available, today's most important questions are: How can these technologies be integrated into the European energy system? What are the costs and benefits of such a strategy? What are the major bottlenecks and obstacles to such a development? What measures are necessary to support this development? In the book a "sustainable scenario" and a "fair-market scenario" are developed as a means to demonstrate that concepts for a sustainable future European energy supply are feasible.