A proton exchange membrane (PEM) fuel cell is an energy converting system generating electricity by oxidation of hydrogen and reduction of oxygen with water and heat as the only waste products. Despite the huge market potential of the fuel cell, its performance and cost must be improved significantly before constituting a viable market. One of the major problems of current fuel cells is water management:​ at energy demanding conditions where the cell is operating at high current densities, excessive water produced restricts the access of reactant gases and hence reduces the performance of the cell. To improve water management, it is necessary to study water transport mechanisms in the internal network of the cell, especially in the porous gas diffusion layer (GDL) through which transport of electrons, reactant gases, and water occurs. In this thesis, fluid flow through the GDL is studied experimentally and numerically using fluorescence microscopy and a pore network modeling approach, respectively. The images obtained from the microscope are analyzed to find patterns of flow inside the GDL samples with different hydrophobicity. Three different flow patterns are observed:​ initial invasion, progression, and pore-filling. The observations show that liquid water flows into the majority of available pores on the boundary of the hydrophilic (untreated) GDL and several branches segregate from the initial pathways. For the hydrophobic (treated) GDL, however, a handful of boundary pores are invaded and the original pathways extend toward the other side of the medium with minimum branching. In addition to flow visualization, the experimental setup facilitates the precise measurement of pressure and time of breakthrough which are used as boundary condition and the validation criterion for the numerical simulation, respectively. The numerical model, developed based on an invasion percolation algorithm, is used to study the effects of GDL hydrophobicity and thickness on the flow configuration and breakthrough time as well as to determine the flow rate and saturation in different GDL samples. The developed model can be used to optimize the GDL properties for designing porous medium with an effective transport characteristic