Polymer electrolyte membrane fuel cells (PEMFCs) use a porous electrode structure usually referred to as a membrane-electrode assembly (MEA). A schematic cross-section of a standard MEA with typical dimensions listed is shown in Figure 24.1. PEMFC operation involves multiphase transport of gaseous species and liquid water through the porous layers of the MEA. Because of the multiphase aspects involved, a full mechanistic understanding of transport in fuel cell electrodes requires knowledge of capillary properties. šis chapter will outline some important and relevant concepts of capillarity and wettability, describe the relevant experimental techniques that have been developed to study wettability and capillary behavior in electrode materials and discuss the most recent œndings concerning the ežect of degradation and aging on wettability.