The advent of solid‐state polymer electrolytes for application in lithium batteries took place more than four decades ago when the ability of polyethylene oxide (PEO) to dissolve suitable lithium salts was demonstrated. Since then, many modifications of this basic system have been proposed and tested, involving the addition of conventional, carbonate‐based electrolytes, low molecular weight polymers, ceramic fillers, and others. This Review focuses on ternary polymer electrolytes, that is, ion‐conducting systems consisting of a polymer incorporating two salts, one bearing the lithium cation and the other introducing additional anions capable of plasticizing the polymer chains. Assessing the state of the research field of solid‐state, ternary polymer electrolytes, while giving background on the whole field of polymer electrolytes, this Review is expected to stimulate new thoughts and ideas on the challenges and opportunities of lithium‐metal batteries. Safety net: Polymer electrolytes are a safe alternative to conventional liquid electrolytes in lithium batteries. Their main drawback is low ionic conductivity at room temperature. The most promising solution for this issue is incorporation of ionic liquids, which enhance the performance without decline in safety. This Review elucidates the interactions in these ternary polymer electrolytes and their performance in lithium‐metal polymer batteries.