The past decade has witnessed the explosion of research on atomically dispersed metal catalysts or "single-atom catalysts (SACs)". The interest is driven by the desire to mimic the homogeneous pathway for extremely high activity and chemoselectivity on durable, processfriendly heterogeneous supports. Particularly, SACs are advantageous in their maximum metal utilization efficiency and atomically precise coordination compared to other subnanometric variants (clusters, nanoparticles, etc.). Extensive efforts have delivered significant progress in the areas of synthesis, characterization, applications, and mechanism insights of SACs. However, downsizing to the single-atom level brings its own set of challenges. Conventional wisdom suggests that active metal single atoms tend to agglomerate in working conditions to minimize the surface energy, which is generally due to two mechanisms at play: site-migration and coalescence of entire nanoparticles and Ostwald ripening with the motion of atomic species from smaller to larger particles. In both mechanisms, an emergence of larger particles will usually lead to the loss of catalytic activity and selectivity. Many strategies are proposed to prevent the aggregation of SACs by regulating the metal-substrate interaction and more recently, to reverse the aggregation by external stimuli. Such reversible (dynamic) aggregation is highly desirable for the application in complex reactions that require activation of two (or more) reactants, where the single-site mechanism of SACs is inefficient compared to the dual or multi-site mechanism of clusters or nanoparticles. Understanding the dynamic structural evolution of SACs is the key to establishing the structure-function relationship for subnanometric catalysts composed of single atoms and clusters. Despite its significance, monitoring the dynamic structural evolution of SACs is not easy. The challenges arise from the inherent heterogeneity of supported surfaces associated with binding sites (corners, edges, and faces), defects (vacancies, terraces, and grain boundaries), and amorphous structures (hydrated layers, amorphous supports, and so on). Most