Abstract: Polymer electrolyte membrane (PEM) fuel cells that are modeled and constructed as a differential reactor enable an examination of the kinetics and dynamics associated with the operating fuel cell. The differential reactor bypasses more complex two- and three-dimensional integral reactors and simplifies the fuel cell to a one dimensional system where spatial gradients are removed. The balance between water production and water removal in the differential reactor gives rise to ignition/extinction phenomena and multiple steady states. This phenomenon is a direct result of the polymer electrolyte membrane's role as a reservoir for water. A remarkable analogy between water balance in the differential fuel PEM fuel cell and the energy balance in the classical exothermic stirred tank reactor can be established. In the initial chapters of this thesis, the rationale behind the design of the PEM fuel cell as a differential reactor is described. A mathematical model of the PEM fuel cell as a stirred tank which incorporates four of the key operating parameters: load resistance, fuel cell temperature, inlet hydrogen flow rate, and inlet oxygen flow rate, successfully captures the ignition/extinction phenomena. Changes in the water inventory as a result of a change in any operating parameter will alter the membrane resistance. This affects the rate of water production and ultimately will affect ignition/extinction. In the later chapters of this thesis, the differential reactor is used as a building block to model the integral type fuel cell reactors. The segmented anode parallel channel PEM fuel cell was developed to provide insight into the inner workings of the fuel cell. Specifically, this version of the fuel cell enabled individual current measurements within each segment which led to a spatial observation of the ignition front. A model of this segmented anode parallel channel fuel cell based on coupled stirred tanks in series is also presented at the end. In addition to the effects of the operating parameters on the fuel cell operation, flow effects (co- and counter-current), and also configuration effects were studied.