A mathematical framework for the design of proton exchange membrane fuel-cell stacks has been established. Sensitivity analysis on the steady-state model of a single fuel cell identified the inlet gas flow rates, the operating pressure, and the temperatures as the important operating parameters. A performance model of a flow field is developed to describe the flow of the reactant gases across a single cell. The reactant flow in the stack manifold is modeled to provide an engineering estimate of the variations in gas flow to the individual fuel cells in the stack. The single-cell model is integrated with the performance model of the flow field and the stack-gas manifold to predict the operational characteristics of the fuel-cell stack.