Recent realization of high sodium‐ion conductivities (>10−2 S cm−1) in inorganic solid electrolytes (ISEs) at room temperature will certainly trigger a boom in all‐solid‐state sodium batteries (ASS‐SBs). However, their electrochemical stable windows and compatibility to high capacity/voltage electrodes are unsatisfactory. Developing ideal ISEs that deliver high Na+ ion conductivities, good electrochemical/chemical stability, and compatible electrode/ISE interface is key for the success of high‐performance ASS‐SBs. In this review, focus is mainly on the fundamentals and strategies to optimize ASS‐SB performances from the aspects of ISE and interface, and note that interfacial issues are also ISE‐related. The latest progress in ISEs, including fundamentals of the sodium‐ion conduction mechanism, key parameters dominating the Na+ ion conduction in terms of crystal structure, lattice dynamics, point defects, and grain boundaries, and prototyping strategies for cell design, are elaborated from the perspectives of material and defect chemistry. The key challenges and future opportunities are discussed, and rational solutions are provided. This review discusses the fundamentals of sodium ionic conduction in inorganic solid electrolytes (ISEs), identifies the key parameters dominating the sodium ionic conduction, and proposes prototyping strategies for both solid electrolyte and full cell design, from the perspectives of materials and defect chemistry. The key challenges and future opportunities for all‐solid‐state sodium batteries using ISEs are also discussed in detail.