The variable energy sources drive the need for flexibility to restore a system’s energy balance. The flexibility sources, i.e. demand side response, dispatchable power plants, storage and interconnection, can respond to restore that balance. Electric vehicles, including plug-in EVs and fuel cell electric vehicles (FCEVs), have a huge potential to play an important role in future energy systems. EVs and FCEVs can be used to discharge electricity to the grid, and when aggregating the power of a large number of vehicles, they can function as dispatchable power plants. Plug-in EVs can adapt their charging behaviour to the needs of the power system operator, and similarly they can act as storage by charging their batteries for example, when there is a surplus of renewable energy. Fuel cell cars (FCEVs), while parked, can produce electricity more efficiently than the present electricity system and with useful ‘waste’ products, heat and fresh water. In terms of technology, the energy production system “Car as Power Plant” can be envisaged as a fleet of fuel cell vehicles, where cars, while parked (over 90% of the time), can produce with the fuel cell electricity, heat and fresh water that can be feed into the respective grids. The Car as Power Plant system with FCEVs has the potential to replace all electricity production power plants, creating a flexible detachable decentralized multi-modal energy system. This chapter will address the role of electric mobility in the future energy systems in general and its role in demand response and in flexible generation.