This book presents the utilisation of a PEM fuel cell as a main power source in a distributed generation system, particularly for future electric vehicle applications. The fuel cell, known as a high specific energy source at the present time, is one of the well-known alternative sources of electric power generation when given consideration for decreasing oil consumption and hazardous CO2 emissions. Nevertheless, one of the main weak points of the fuel cell is its time constants dominated by temperature and fuel delivery system (pumps, valves, and in some cases, a hydrogen reformer). As a result, fast load demand will cause a high voltage drop in a short time, recognised as a fuel starvation phenomenon. One must limit the fuel cell current (or power) slope to prevent fuel starvation problems, to improve its performance and lifetime. Therefore, to employ a PEM fuel cell in dynamic applications, the electrical system must have at least an auxiliary power source to improve system performance when electrical loads at a DC bus demand high energy in a short time, as well. The possibilities to use a supercapacitor or battery bank as an auxiliary source with a fuel cell main source is presented in detail. The very fast power response and high specific power of a supercapacitor or high specific energy of battery can complement the slower power output of the main source to produce the compatibility and performance characteristics needed in a load.