The hydrogen polymer electrolyte membrane (PEM) fuel cell provides enormous potential for a future with clean energy. However, due to cost and inefficiency barriers, PEM fuel cells have not yet reached widespread commercial adoption. Mass transport limitations arising from liquid water flooding in low temperature PEM fuel cells and acid leaching in high temperature PEM fuel cells leads to inefficiencies. If these issues become resolved, smaller and more reliable devices could be produced at a lower cost. Mass transport limitations can be minimized through the development of optimized materials, which have tailored pore structures, connectivities, conductivities, and surface wettabilities. The porous materials in PEM fuel cells could be customized for mass transport with detailed information about their structure and the dominating mass transport mechanisms that result from these structures. In this talk, X-ray based visualization techniques, such as microcomputed tomography, synchrotron X-ray radiography, and neutron radiography, for investigating multiphase transport in PEM fuel cells will be discussed.