AbstractSolid‐state batteries (SSBs) are considered to be one of the most promising candidates for next‐generation energy storage systems due to the high safety, high energy density and wide operating temperature range of solid‐state electrolytes (SSEs) they use. Unfortunately, the practical application of SSEs has rarely been successful, which is largely attributed to the low chemical stability and ionic conductivity, ineluctable solid‐solid interface issues including limited ion transport channels, high energy barriers, and poor interface contact. A comprehensive understanding of ion transport mechanisms of various SSEs, interactions between fillers and polymer matrixes and the role of the interface in SSBs are indispensable for rational design and performance optimization of novel electrolytes. The categories, research advances and ion transport mechanism of inorganic glass/ceramic electrolytes, polymer‐based electrolytes and corresponding composite electrolytes are detailly summarized and discussed. Moreover, interface contact and compatibility between electrolyte and cathode/anode are also briefly discussed. Furthermore, the electrochemical characterization methods of SSEs used in different types of SSBs are also introduced. On this basis, the principles and prospects of novel SSEs and interface design are curtly proposed according to the development requirements of SSBs. Moreover, the advanced characterizations for real‐time monitoring of interface changes are also brought forward to promote the development of SSBs.