In contrast to averaging spectroscopic methods, microscopies provide spatially resolved information on different catalyst positions in every image frame, with spatial and time resolution depending on the microscope. Transmission electron microscopy (TEM) has long been used to determine particle size distributions (histograms). Recent advances in aberrationcorrected scanning transmission electron microscopy (AC-STEM), coupled with electron energy loss spectroscopy (EELS), allow the verification of the chemical identity of constituent elements with atomic resolution. Visualizing SACs is also essential for understanding the formation, coordination environment, and stability of the catalyst, mainly when the catalytic performance is dominated by dynamic aggregation into other subnanometric species. The most well-known example would be the direct observation of transforming nanoparticles Owing to the coexistence of two competitive atomization and agglomeration processes, initial sintering will occur, leading to a steady increase in the diameter of Pd nanoparticles from room temperature to 900 °C. However, these nanoparticles would eventually transform into single atoms to minimize the ΔG of the system. Once the temperature increased to 1,000 °C, agglomeration and atomization accelerated. Small Pd nanoparticles vanished in situ at 1,000 °C after 36 s (Figure ) due to the capture of mobile Pd atoms by N defects on the substrate. Meanwhile, the sintered Pd nanoparticle (6.5 nm in diameter) underwent thermal motions within the substrate and downsized to 4.0 nm at 136 s and 1.9 nm at 150 s by Pd coordination with the N defects before final atomization to single atoms at 162 s. This was supported by the highly exothermic (-3.96 eV) process for forming a Pd-N4 SAC from the decomposition of the Pd10 cluster in Figure . Such transformation required a moderate energy barrier of 1.47 eV to be overcome, which was much higher than the diffusion energy of a single Pd cluster for sintering (0.58 eV). Hence, sintering was dominant at relatively low temperatures, and atomization dominated at high temperatures (900 ~ 1,000 °C). Similarly, the dynamic and reversible transformation between carbon-supported single Pt atoms and their agglomerates under redox conditions was monitored by in situ TEM. In another report, the dynamic process of nanoporous Au to catalyze methane pyrolysis was monitored by in situ TEM, demonstrating the release of Au single atoms by partial disintegration of nanoporous Au surfaces. DeRita et al. also systematically investigated the movement of Pt SACs dispersed on anatase TiO2 under different reducing and oxidizing conditions via in situ AC-STEM. The variation in local coordination strongly influenced the chemical reactivity for CO oxidation. We should point out that in situ TEM observation under vacuum is only suitable for certain reactions, primarily pyrolysis of SAC precursors and gas-phase reactions. A complete picture of a catalyst should include operating conditions and investigations into degradation. However, replicating the catalytic environment poses challenges in losing spatial resolution or scattered electrons due to equipment limitations. A typical challenge lies in the TEM observation of SACs in a liquid environment, which is more valuable for applications in (photo)electrochemical and organic catalysis. Conventional liquid cells are made with silicon nitride (SiNx) windows and are only suitable for observation in nanoparticle formation, dissolution, and shape changes. with the predominant adatoms located on the surface rather than substituted in the MoS2 lattice. In summary, microscopy always encounters questions of whether these observations are representative of the whole sample. It should be cross-referenced by other spectroscopic techniques to exclude the influence of artifacts and beam-induced damage generated by the experimental conditions. Apart from this, spectroscopic data at atomic resolution tends to be noisy from the low signal-to-noise ratio, urging future technological development.