Publisher Summary A fully developed control system consists of thermocouples, pressure transducers, methanol/hydrogen sensors, and mass flow controllers, which measure and control data using a data acquisition program. The fuel cell catalyst, membranes, and flow field plates are important areas to be considered for fuel cell design, modeling, and improvement. This chapter focuses on modeling these important subsystems. Fuel cell system designs range from very simple to very complex depending upon the fuel cell application and the system efficiency desired. A fuel cell system can be very efficient with a few plant components. Humidifiers, fans, blowers, compressors, turbines, and pumps are the plant components that are reviewed. Other additional components often found in the fuel cell plant include: heat exchangers, pumps, fans, blowers, compressors, electrical power inverters, converters and conditioners, water handling devices, and control systems. Most of the fuel cell system components are used to distribute air and hydrogen flows into and out of the fuel cell stack. A series of quick models aid the fuel cell stack designer to make the best design decisions. The larger the fuel cell stack, the more complex the fuel cell plant subsystem. Modeling the fuel cell system efficiency and output allows the system designer to be more efficient when creating new systems.