A fuel cell stack does not work by itself, but its operation requires the utilization ofseveral auxiliary sub-systems to permit an efficient and reliable electric powerproduction. In this chapter all these components are described, evidencing theirrole in determining the fuel cell stack operation. In particular, this discussion isaimed at verifying if and how a fuel cell system (FCS) could really match allvehicular requirements, evidencing the parasitic losses of auxiliary componentsand dynamic performance in terms of response to start-up, frequent stop and gophases, fast load changes, variable power levels. The interaction between FCSperformance and management strategies necessary to guarantee an optimal stackoperation in automotive applications is discussed, with a brief overview of FCScosts.