This review focuses on the solid electrolyte interphase (SEI) in lithium metal batteries (LMBs), which is crucial for inhibiting dendrite growth and enhancing cycling performance. The mechanisms of SEI formation and models of its structure are discussed, emphasizing the critical factors such as electrolyte components, temperature, and current density. Various characterization methods, including surface chemistry, morphology, electrochemical properties, and dynamic characteristics, are reviewed to understand the SEI layer. The review also covers efficient methods to modify the SEI layer, such as using new electrolyte systems, additives, and ex-situ-formed protective layers, as well as electrode design. Despite recent advancements, robust and precise routes for SEI modification and a deeper understanding of the connection between structure and electrochemical performance are still needed. A multidisciplinary approach is essential to enable the formation of robust SEI for highly efficient energy storage systems.This review focuses on the solid electrolyte interphase (SEI) in lithium metal batteries (LMBs), which is crucial for inhibiting dendrite growth and enhancing cycling performance. The mechanisms of SEI formation and models of its structure are discussed, emphasizing the critical factors such as electrolyte components, temperature, and current density. Various characterization methods, including surface chemistry, morphology, electrochemical properties, and dynamic characteristics, are reviewed to understand the SEI layer. The review also covers efficient methods to modify the SEI layer, such as using new electrolyte systems, additives, and ex-situ-formed protective layers, as well as electrode design. Despite recent advancements, robust and precise routes for SEI modification and a deeper understanding of the connection between structure and electrochemical performance are still needed. A multidisciplinary approach is essential to enable the formation of robust SEI for highly efficient energy storage systems.