This review describes the advantages and characteristics of employing polymer electrolytes in solid-state lithium-ion batteries. Criteria for an ideal polymer electrolyte and the differences between polyelectrolytes are discussed. The emphasis of this article is on plasticized or gelled electrolyte systems. Hence, the review focuses on four plasticized systems which have received particular attention from a practical viewpoint, i.e., poly(ethylene oxide) (PEO)-, poly(acrylonitrile) (PAN)-, poly(methyl methacrylate) (PMMA)-, and poly(vinylidene fluoride) (PVdF)-based electrolytes. Some critical concepts and points associated with this emerging technology that still require attention are discussed in the final part of the review.