Electrochemical energy storage devices are pivotal in achieving “carbon neutrality” by enabling the storage of energy generated from renewable sources. To facilitate the development of these devices, it is important to gain insight into the underlying the single-/multi-electron transfer process. This can be achieved through in-time detection under operational conditions, but there are limited tools available for monitoring electron transfer under operando conditions. Electron paramagnetic resonance (EPR) is a powerful technique that can meet these expectations, as it is highly sensitive to unpaired electrons and can detect changes of paramagnetic centres. Despite the long history of in situ electrochemical EPR research, its potential has been surprisingly underutilized due to the need for strict operando cell design under special testing conditions. This review comprehensively summarizes recent efforts to understand energy storage mechanisms using in situ/operando EPR, with the aim of drawing researchers’ attention to this powerful technique. After introducing the fundamental principles of EPR, we describe the critical advances made in detecting batteries using operando EPR, along with the remaining challenges and opportunities for future development of this technology in batteries. We emphasize the need for strict operando cell design and the importance of designing experiments that closely mimic real-world conditions. We believe that this review will provide innovative solutions to solve tough problems that researchers may encounter during their battery research, and ultimately contribute to the development of more efficient and sustainable energy storage devices.