Polymer electrolytes, generally, are all those materials where a supramolecular system is doped with ions and presents a significant conductivity. Following the 12th and most recent International Symposium of Polymer Electrolytes (ISPE-12), this historical and critical review summarizes the main developments in the fundamental science and applications of this family of materials since their establishment approximately 40 years ago. The history of polymer electrolytes is briefly outlined and the main families are reviewed in the framework of a tentative classification scheme. The applications of polymer electrolytes are discussed, highlighting the role of the electrolyte materials and reporting the most widely accepted performance and durability benchmarks in the various fields. Finally, a statistical analysis of the available scientific literature is presented to identify the most relevant current trends and future possibilities in the diversification and growth of worldwide research on this topic.