Growing environmental concerns have led to a huge interest in renewable energy sources. Fuel cells have the potential to be one of these sources. They are very well suited for automotive and working machine applications, where they can replace internal combustion engines. Unfortunately, the load profiles in such applications contain a large number of high transients including regenerative braking peaks. In order to maximize the fuel cell lifetime and to enable the ability to recover braking energy, the fuel cell has to be paralleled with energy storage systems like batteries and supercapacitors or both. The resulting system is called a hybrid fuel cell powertrain. A hybrid fuel cell powertrain can be constructed without power electronics but only in special applications. Practical hybrid powertrains require the use of power electronic converters from which DC/DC conversion is perhaps the most important. The dynamic behaviour of every energy source associated with the powertrain has to be understood in order to design proper DC/DC converters. Especially fuel cells are low-voltage high-current devices requiring the use of high-power DC/DC converters with low input-current ripples. Various topologies have been proposed to fulfil this requirement and the requirements of energy storage systems as well. In addition to the topological issues, the control design of the converters is equally important for assuring stable powertrain operation and sufficient transient dynamics. The results obtained from the simulations imply that a well-behaving powertrain can be realized, provided that the control loops are designed properly. The results emphasize the need for proper sizing of the powertrain components and the need for choosing correct powertrain configuration with appropriate voltage levels. It can be determined from the results, that even though a powertrain equipped with power electronic converters is more complex than a powertrain without such devices, it has very clear and strong advantages. /Kir10