The determination of catalytically active sites in subnanometric metal catalysts is challenging owing to their structural dynamism. Particularly, they may have completely different activities toward the same reaction. It has been reported that, in some reactions, SACs are the active species, while clusters and nanoparticles are not. In such events, a sharp increase in catalytic turnover will be observed at low metal content when the majority of metal centers become spatially isolated (Ullmann reaction in Figure ). If nanoparticles can also catalyze the reaction, the activity will rise more gradually with increasing dispersion until it reaches a plateau due to the maximized atom utilization (Pt1/FeOx for nitrostyrene hydrogenation). For those reactions where SACs are ineffective (such as Pd1/Fe3O4 for styrene hydrogenation), size reduction will lead to diminished activity as metal clusters are no longer present in the sample. Additionally, an induction period may appear when the "real" catalyst is not initially added into the system, while transformation into inactive species results in a decline in activity in a prolonged reaction period. To further understand the imperative role of structural dynamism, a correlation between the coordination number of Pt-Pt and the corresponding Tafel slope in hydrogen evolution reaction (HER) was established for a series of Pt-based catalysts in Figure . Atomically dispersed Pt SACs possess a substantially high Tafel slope and undergo restructuring into tiny clusters due to weakened Pt-N bonding under cathodic potentials. This leads to drastically decreased Tafel slopes and a transition from the Volmer-Heyrovsky pathway to the Volmer-Tafel pathway when the particle size increases to 2 nm or even larger. Such tiny clusters are responsible for the improved HER activity and stability rather than the initial Pt SACs. Likewise, the electrochemically reconstituted Cu4 clusters from Cu1/CeO2 are real active sites for electrocatalytic urea synthesis due to favorable C-N coupling reactions and urea formation. An in situ formed Cu(I)/Cu(0) mixture from the initial Cu1/TiO2 can effectively promote the generation of CH4 in the photocatalytic CO2 reduction. The breakage of the Rh-Rh bond into the mononuclear complex in supported Rh nanoparticles could be facilitated by the iodine radical and CO molecules in the working condition. Conversely, the redispersion of clusters into single atoms could also be beneficial. As shown in Figure , the on-stream activity of 1Pt/Fe2O3 nanoparticles displayed a monotonic increase in 4 h at 700 o C to reach a 65% conversion for methane oxidation. This was accompanied by the disappearance of nanoparticles and the formation of atomically dispersed Pt in the spent catalyst in Figure . Such phenomenon is more likely to occur in thermally stable SACs for gas-phase reactions at high temperatures, including the Pd-N4 SAC for semi-hydrogenation of acetylene, atomically dispersed CuOx for deNOx reactions (NO + CO or NH3 + NO + O2), 7