Polymerized ionic liquids, poly(ionic liquid)s or polymeric ionic liquids, all abbreviated as PIL is the polymeric form of ionic liquids. They have half of the ionicity of ionic liquids since one ion is fixed as the polymer moiety to form a polymeric chain. PILs have a similar range of applications, comparable with those of ionic liquids but the polymer architecture provides a better chance for controlling the ionic conductivity. They have extended the applications of ionic liquids for designing smart materials or solid electrolytes.