Publisher Summary This chapter describes the design of fuel cell stack. A fuel cell stack consists of a multitude of single cells stacked up so that the cathode of one cell is electrically connected to the anode of the adjacent cell. The first step in designing a fuel cell stack is to determine its active area and number of cells in the stack. When a stack is designed for an application, the design inputs come from the application requirements, such as desired power output, desired or preferred stack voltage or voltage range, desired efficiency, and volume and weight limitations. In some stack configurations, humidification of one or both reactant gases is included in the stack, either in a separate stack section or between each cell. In both cases water is used for both cooling and humidification, and the heat generated in each of the active cells is used for humidification. In another configuration, water passes between each cell separated by a porous graphite plate from the reactant gases. Once the reactant gases enter the individual cell they must be distributed over the entire active area. To maintain the desired temperature inside the cells, the heat generated as a by-product of the electrochemical reactions must be taken away from the cells and from the stack.