Power electronics will play a key role in future fuel cell energy conversion applications. Section 4.13.1 considers a terrestrial fuel cell application discussing the interconnection of fuel cells to electricity grid networks. The fuel cell energy source has a variable output voltage range with load that impacts on the optimization of the conversion electronics which will be illustrated by consideration of a system design example. Electric vehicles are set to play a prominent role in addressing the energy and environmental impact of an increasing road transport population by offering a more energy efficient and less polluting drive-train alternative to conventional internal combustion engine (ICE) vehicles. Given the energy (and hence range) and performance limitations of electro-chemical battery storage systems, hybrid systems combining energy and power dense storage technologies have been proposed for vehicle applications. Sections 4.13.2 and 4.13.3 will discuss some of the application issues associated with the implementation of hybrid energy sources for electric vehicle traction systems, in particular fuel cells and other storage devices. In section 4.13.2 a typical automotive fuel cell system is characterized and the application thereof in a vehicle power-train is discussed in Section 4.13.3.