Neutron depth profiling (NDP) is a non?destructive, operando technique that directly quantifies lithium distribution and transport in all?solid?state batteries (ASSBs) by exploiting the 6Li(n, α)3H nuclear reaction. Conventional techniques (e.g., X?ray methods) rely on interactions with extranuclear electrons and thus struggle to detect lithium due to its low electron density. By contrast, NDP exploits the direct nuclear reaction, enabling unambiguous localization of 6Li atoms. Consequently, NDP provides quantitative, depth?resolved monitoring of lithium concentration profiles along the battery thickness direction with a typical depth resolution of tens of nanometers. This review systematically summarizes recent progress in applying NDP to four key aspects of ASSBs: electrolytes, electrodes, interfaces, and thin?film full batteries. In electrolytes, NDP has revealed electronic?conductivity?induced random dendrite nucleation and reversible short?circuit phenomena. In electrodes, it has guided 3D framework designs, validated surface halogenation for Si anodes, and verified longitudinal Li gradients in composite cathodes. In interfaces, NDP has visualized space?charge layers, quantified lithiophilic coating effects, and provided early detection of short?circuit failure. In thin?film batteries, NDP has tracked Li?ion transport in near real time using isotopic tracers, identified Si migration as a degradation mechanism, and pinpointed rate?limiting interfaces in combination with other techniques. Beyond these applications, the inherent advantages and current limitations of NDP—such as its sensitivity to 6Li isotope, the trade?off between depth resolution and counting statistics, and the requirement for flat sample surfaces—are critically discussed. This review establishes NDP as an indispensable tool for understanding and engineering lithium transport in ASSBs, and outlines future directions including higher?flux neutron sources, improved detector systems, and integration with complementary in situ probes.