Proton (H-1) NMR microscopy is used to investigate in-situ the distribution of water throughout a self-humidifying proton-exchange membrane fuel cell, PEMFC, operating at ambient temperature and pressure on dry H-2(g) and O-2(g). The results provide the first experimental images of the in-plane distribution of water within the PEM of a membrane electrode assembly in an operating fuel cell. The effect of gas flow configuration on the distribution of water in the PEM and cathode flow field is investigated, revealing that the counter-flow configurations yield a more uniform distribution of water throughout the PEM. The maximum power output from the PEMFC, while operating under conditions of constant external load, occurs when H2O(I) is first visible in the H-1 NMR image of the cathode flow field, and subsequently declines as this H2O(I) continues to accumulate. The H-1 NMR microscopy experiments are in qualitative agreement with predictions from several theoretical modeling studies (e.g., Pasaogullari, U.; Wang, C. Y. J. Electrochem. Soc. 2005, 152, A380-A390), suggesting that combined theoretical and experimental approaches will constitute a powerful tool for PEMFC design, diagnosis, and optimization.