This chapter is devoted to polymer electrolytes for use in membrane electrode assemblies of proton exchange membrane fuel cells. This chapter starts with a discussion of structure–property relationships of polymer electrolytes used as proton exchange membranes, in terms of water uptake, conductivity, and methanol permeability. The second part discusses polymer electrolytes used as electrode ionomers, specifically the structural effects of ionomer composition, ion exchange capacity, hydrophobicity, and dispersing solvent on fuel cell performance. The issue of interfacial compatibility between membrane and electrode is also discussed. ; peer reviewed: yes ; NRC Pub: yes