Nanofibers have unique functionalities based on their nanoscaled cross-section, high specific surface area, and high molecular orientation inside the fibers. These functionalities can be improved by controlling their diameter, surface and internal structures. The introduction of ion-exchange sites on the surface and/or inside the nanofibers provides novel high-functional ion-exchangers with high surface areas, high ion-exchange and adsorption capacities, and high ionic conductivities. The present review highlights the fundamental aspects of ion-exchange nanofibers (i.e. their size-dependent unique properties and production methods) and their recent advances in the environment and energy applications (i.e. separation/purification processes and fuel cells). (c) 2018 Society of Chemical Industry