This chapter discusses the overall mitigation potential at various cost levels, the contributions of the sectors, and the question of where the potential is located. It concludes that the mitigation potential is big enough to bring global emissions back to current levels by 2030. With this potential stabilization of atmospheric concentrations of greenhouse gases at levels of 450 ppm CO2-eq is still within reach, provided that the potential in developing countries is also tapped. Geo-engineering is not needed, and that is comforting, because the risks and uncertainties of such planetary experiments are huge. In all countries substantial potential exists with so-called negative costs (i.e. where investment is profitable), but costs differ as a result of strong differences in national circumstances. Cost for the economy as a whole is limited when cheap options are implemented first. On average annual economic growth rates will not be reduced by more than a few tenths of a percentage point, and that is without taking co-benefits for energy security, health, and employment into account. Investments required for implementing the reduction options will have to shift strongly to efficiency of energy use and low carbon energy, and are bigger than those without stringent climate policy, but are compensated by much lower energy costs. Implementing low carbon technology rapidly in developing countries is crucial to controlling climate change. National priorities in developing countries for modernization, energy security, and trade are the main drivers. Governments in the North and South should remove obstacles and create the right conditions.