The first chapters of this report discussed the initial setting (Chapter 2) and the technological and sustainable potentials of present global energy sources (Chapter 3). For several reasons, today's global energy system must be considered non-sustainable. In particular, its climate impacts jeopardize the lifesupport systems on which human existence depends, air pollution and the unsustainable use of biomass generate substantial health problems and some 2,000 million people still lack access to modern forms of energy.