Eco-efficiency still is the most important option for harmonizing economic growth and environmental goals, for example, as formulated in the UN SDGs 2015. Eco-efficiency increases provide the chance to continue growth of output (GDP) and to reduce input (natural resource consumption) at the same time, that is, delinking (decoupling) the two variables. If eco-efficiency increases significantly, doubling of GDP within three decades could be combined with halvening of resource inputs ("factor 4"). However, there is no empirical evidence for such high rates of eco-efficiency increase. An "efficiency revolution" would be necessary but has not materialized yet. Moreover, increases in ecoefficiency tend to reduce cost of production and prices of productshence stimulate output and resource consumption. This expansive effect is called "rebound effect." It has been analyzed already in 1866 by W. St. Jevons (the Coal Question). Empirical studies today find significant rebound effects, often between 25% and 100%. The larger it is, the less can eco-efficiency contribute to sustainable development (reduction) goals. Eco-efficiency increases and their expansive effect are the fundament of "green growth." Eco-efficiency stimulates the economy and increases growth rates. This is good for jobs and incomes but not for the environmental impact. The rebound effect can be reduced if eco-efficiency increase is combined with quantity (caps) or price (tax) measures. However, the rebound effect reduces the practical relevance of eco-efficiency increases. Sustainability policy should therefore not only be based on efficiency but has to consider the second option, sufficiency, too. Eco-efficiency and Sustainable Development Increase of eco-efficiency is the dominating answer to almost all challenges in the context of sustainable development. The basic idea is simple: Environmental impact I (pollution) depends on two components: the size of GDP and the pollution (resource consumption) which is necessary to produce one unit of GDP: