Although Naon membranes were primarily developed for the chloro-alkali processes by DuPont in the 1960s, they did demonstrate unexpected applicability to polymer electrolyte membrane fuel cells (PEMFCs) because of their high proton conductivity and inherent chemical, thermal, and oxidative stability. Moreover, they were widely used as polymer electrolyte membranes (PEMs), a key component in PEMFCs, and considered as the de facto benchmark of PEMs. The PEM functions as a protonic conductor, an electronic insulator, and a separator for the two-reactant gases. Because Naon membranes play such important roles in PEMFCs, it is necessary to gain an insight into the nanostructures in Naon membranes, which can be useful in understanding the relationships among their chemical structure, morphology, and macroscopic physicochemical properties (Figure 11.1).