Owing to the use of solid electrolytes instead of flammable and potentially toxic organic liquid electrolytes, all solid-state lithium batteries (ASSLBs) are considered to have substantial advantages over conventional liquid electrolyte based lithium ion batteries(LIBs) in terms of safety, energy density, battery packaging, and operable temperature range. However, the electrochemistry and the operation mechanism of ASSLBs differ considerably from conventional LIBs. Consequently, the failure mechanisms of ASSLBs, which are not well understood, require particular attention. To improve the performance and realize practical applications of ASSLBs, it is crucial to unravel the dynamic evolution of electrodes, solid electrolytes, and their interfaces and interphases during cycling of ASSLBs. In situ transmission electron microscopy (TEM) provides a powerful approach for the fundamental investigation of structural and chemical changes during operation of ASSLBs with high spatio-temporal resolution. Herein, recent progress in in situ TEM studies of ASSLBs are reviewed with a specific focus on real-time observations of reaction and degradation occurring in electrodes, solid electrolytes, and their interfaces. Novel electro-chemo-mechanical coupling phenomena are revealed and mechanistic insights are highlighted. This review covers a broad range of electrode and electrolyte materials applied in ASSLBs, demonstrates the general applicability of in situ TEM for elucidating the fundamental mechanisms and providing the design guidance for the development of high-performance ASSLBs. Finally, challenges and opportunities for in situ TEM studies of ASSLBs are discussed.< Réduire