The development of PEM fuel cell systems can be further improved, if the interactions between the different development levels are considered. The IPEK X-in-the-loop approach allows connecting the operating conditions measured locally at the fuel cell components, like the bipolar plate with data from the drivetrain and the fuel cell system. Measurement data taken from micro sensors can be transmitted to control the fuel cell system components. The mixed virtual-physical approach allows emulating different types of components instantly and facilitates the optimization of fuel cell components and operating strategies. With this development method the fuel cell and the fuel cell system components as well as the operating controls can be improved with respect to power dynamics, power density and lifetime.