In this talk, a macro-homogeneous model is developed to explore water management in polymer-electrolyte fuel cells (PEFCs). PEFCs are on the forefront to becoming the energy-delivery devices of the future. However, they suffer from many problems, including low efficiency. This efficiency is directly associated with the issue of the fuel-cell water balance and water management, where PEFC operation is a balance between membrane dehydration and cathode flooding, among other things. The mathematical model developed here takes these issues into account by quantifying the relevant phenomena that occur in the various PEFC sandwich layers: membrane, catalyst layers, and diffusion media. For each of the layers, physical and then mathematical models are developed. Such an undertaking involves the identification of the controlling phenomena and issues that should be considered, and then their mathematical description. The developed layer models are complex enough to account for many of the experimentally observed effects such as Schr√∂der's paradox and two-phase flow, while remaining simple enough to use in an overall PEFC simulation. The proposed model is validated by comparison of computational results with experimental data, including PEFC water balance and polarization curves. The results of the simulations allow for the explanation of observed effects. It is clearly shown how important anode humidification is to overall fuel-cell performance, and also how the effects of flooding can be minimized by changes in system design, such as the inclusion of a micro-porous layer. The model, its analysis, and results provide a route for optimization and mitigation of the technical hurdles currently facing PEFCs.