Catalysts can effectively accelerate the reaction kinetics process and are recognized as the core to realize the conversion and supply of carbon-free energy. However, the active sites of catalysts, especially nanocatalysts, usually undergo dynamic structural evolution under realistic working conditions, which may be induced by various reaction effects such as the applied voltages, electrolytes, or adsorbed intermediates. Therefore, in-depth and systemic insights into the nature of the active sites involved under the working conditions are prerequisites for correlating structure-performance relationships. However, uncovering and identifying active sites under operation conditions are still formidable scientific and technical challenges, which are severely hindered by the complex physical and chemical processes occurring on the active sites. Meanwhile, complementary and important information could be missed by conducting only the conventionally employed ex situ microscopic and spectroscopic measurements. Accordingly, it is highly desirable for us to develop the ever-increasing in situ synchrotron-based techniques to identify the nature of active sites, which renders the rational design of functional catalysts achievable.In this Account, we elaborately highlight the substantial achievements in cutting-edge in situ X-ray spectroscopy (XAS) techniques by presenting several representative carbon-neutral electrocatalytic examples performed in our group to broadcast the principles and virtues of identifying the active sites and tracing intermediate species during electrocatalytic water splitting and electrocatalytic CO2 reduction (ECR). Specifically, we believe that the interactions between the active sites and the support as well as the adsorption behaviors of intermediates are considered to be the important factors that govern the performance in the water splitting reaction. Meanwhile, the structural rearrangement of alloy catalysts driven by the cathodic potential significantly governs the activity and selectivity toward ECR. More importantly, the directions and suggestions for addressing the current limitations and pitfalls that we may encounter in the course of executing in situ experiments are also provided. Accordingly, it is necessary to use multiple in situ synchrotron-based techniques to obtain the comprehensive details. Furthermore, bridging the gap between the real energy devices and half-reactions could help us to approach the realistic mechanism. Beyond that, developing the rapid time resolution of in situ XAS will overcome the challenge of timescale mismatch to capture the faster structural kinetics of catalysts. Therefore, this Account is aimed to increase the awareness and appreciation of conducting in situ investigations on energy conversion reactions, which would be a guideline for us to explore catalytic scopes that remain challenging.