This chapter is a review of polymers used either as electrolytes in lithium batteries or in the form of films at the electrode interface. It includes polymers with various organic and inorganic cores, PILs, and LCs. The role of different composites obtained through blendingBlending, graftingGrafting, cross-linkingCross-linking copolymerizationCopolymerization is studied, as well as that of structure (branched, cyclic, chain architecture effects). The focus is on the state-of-the-art inBatteriesall-solid-state all-solid-state batteriesAll-solid-state batteries. Nevertheless, some notable results are also mentioned for lithium batteries equipped with gel electrolytes, when the amount of liquid added to the polymer (and duly mentioned) is so low that it does not compromise safety. The effects of various components related to polymers on battery performance are also detailed, such as the introduction of plasticizersPlasticizers, fillers, and the choice of lithium salts.