General Motors (GM) conducted two well-to-wheel studies for fundamental clarification on the question of which is the cleanest and most environmentally sustainable source of energy for the mobility of the future. In both studies the complete energy chains were analyzed from fuel production using primary energy to the actual consumption of the fuel in the car, i.e. from the well up to the wheels of the vehicle (well-to-wheel). The aim of the studies was to evaluate total energy consumption on the one hand and, on the other, the total greenhouse gas emissions arising between the production of a fuel and its final use to power an automobile. The North American well-to-wheel study was conducted by GM together with the American Argonne National Laboratory in the U.S. This study, conducted specifically with North American conditions in mind with regard to the energy market as well as the vehicles, serves as an important reference for the world-wide discussion on energy consumption and greenhouse gas emissions caused by traffic. It also provides the basis for the more comprehensive European well-to-wheel study. In the European analysis there were a total of 44 different ways of producing fuel, so-called fuel-paths, and 18 conventional and alternative propulsion concepts projected to 2010 under European conditions. The European study leads to the following conclusions which are to a large extent in line with the results from the American study. 1. Fuel cell vehicles can greatly reduce greenhouse gas emissions, if they run on hydrogen from renewable energy sources, they can eliminate them entirely. 2. Fuel cell vehicles have lower greenhouse gas emissions than gasoline or diesel engines. 3. Hydrogen created by electrolysis generates high greenhouse gas emissions if the electrolysis is done with conventionally generated electricity and practically no emissions if the electricity comes from regenerative sources. 4. Fuel cell vehicles powered by methanol provide no advantage over gasoline or diesel engines. 5. Optimised natural gas vehicles cause around the same amount of greenhouse gas emissions as optimized diesel engines. 6. Combustion engines that run on liquid hydrogen produced from natural gas do not cause any CO2 emissions in the vehicle but they do cause higher overall greenhouse gas emissions than conventional gasoline or diesel engines.