© 2020 Elsevier Ltd Li metal anode has become a research hotspot again because of its high specific capacity (3860 mAh g−1 or 2061 mAh cm−3) and lowest potential among the alkali metals (−3.04 V vs. the standard hydrogen electrode), which is also known as the “return of the king”. The solid-electrolyte interphase (SEI) is a key factor in determining battery safety, power capability, Li deposition morphology, shelf life, and cycle life. In order to solve the instability problem of the inherent SEI, SEI-forming additives in electrolytes and artificial SEI are mainly adopted to improve the anode's comprehensive performance. Here, we highlight the formation principles, function, evolution, and degradation mechanism of the natural SEI. On this basis, the latest research progress on the design and the construction of modified SEI layers toward stable Li metal anode in recent years are systematically reviewed. Furthermore, the advanced theoretical analysis, modeling, and characterization techniques for the SEI are also summarized. Finally, the research status of modified SEI and future research trends for this hot research area are comprehensively evaluated. We believe this timely review would advance further understanding and development of the SEI for Li metal anodes.