Energy efficiency remains one of the most pressing issues in energy policy for the next years. The challenge to stabilize or even decrease the amount of energy consumed while keeping the living standard is one of the most important tasks for energy policy-makers in developed countries. Developing countries, however, find themselves confronted with the need to catch up economically to improve the socioeconomic situation and at the same time with warnings about the environmental effects of growth. This is strongly related to the question of the ecological footprint and of mitigation of greenhouse gas (GHG) emissions which are provoked by energy use and are held responsible for climate change and its already experienced impacts.