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Solid‐state lithium batteries are considered promising energy storage devices due to their superior safety and higher energy density than conventional liquid electrolyte‐based batteries. Lithium aluminum germanium phosphate (LAGP), with excellent stability in air and good ionic conductivity, has gained tremendous attention over the past decades. However, the poor interface compatibility with Li anode, slow Li‐ion conduction in thick pellets, and high‐temperature sintering procedure limit the further development of LAGP solid electrolytes in practical applications. This review comprehensively summarizes the crystal structure, Li‐ion conducting mechanism, and various synthesis methods, especially the latest thin‐film preparation approach. The underlying reason for Li/LAGP interfacial instability is identified, followed by several advanced interface engineering strategies, for example, introducing a functional …