All-solid-state batteries (ASSBs) are emerging as a next-generation energy storage technology, offering enhanced safety and energy density compared to conventional lithium-ion batteries. However, critical challenges related to material design and interfacial stability hinder their practical deployment. Advanced synchrotron X-ray and neutron-based techniques have become indispensable for probing the structural, chemical, and morphological evolutions within ASSBs across multiple length scales and under realistic operating conditions. This review provides a comprehensive and critical overview of recent in situ and operando studies of ASSBs enabled by synchrotron and neutron sources. The discussion is organized by the key components and interfaces of ASSBs, highlighting how these techniques elucidate dynamic processes during battery operation. Fundamental principles of the characterization methods are introduced, along with perspectives on future directions. This work aims to guide the rational design of high-performance ASSBs by showcasing how cutting-edge characterization advances can address longstanding challenges.