Single-atom catalysts (SACs) exhibit dynamic aggregation in reactive environments, influenced by thermodynamic and kinetic factors. This dynamic behavior affects catalytic performance, with in situ formed clusters often outperforming single atoms. Operando techniques are essential for understanding structural evolution, while strategies like confinement and defect-engineering regulate aggregation. SACs are thermodynamically unstable, requiring strong metal-support interactions to prevent aggregation. Dynamic aggregation can be reversible or irreversible, impacting activity, selectivity, and stability. Factors such as temperature, voltage, and adsorbates influence this behavior. SACs can switch between active and inactive states under reaction conditions. For example, Rh single atoms on TiO₂ show dynamic coordination changes under different redox conditions. Pt single atoms aggregate in reducing atmospheres but remain stable in oxidizing ones. The stability of SACs is also affected by the support, with reducible oxides like CeO₂ being particularly effective. Multimetallic SACs show unique stability and reactivity due to metal-metal interactions. Dynamic aggregation can enhance catalytic performance, as seen in CO₂ reduction and oxygen reduction reactions. However, it can also lead to catalyst deactivation through sintering or poisoning. Confined SACs in porous materials exhibit distinct dynamic behaviors due to restricted diffusion. Operando techniques like XAS provide insights into the dynamic changes in SACs during reactions. The reversible transformation of SACs into clusters and back again is crucial for maintaining catalytic activity and selectivity. Understanding these dynamics is essential for designing efficient and stable SACs for various catalytic applications.Single-atom catalysts (SACs) exhibit dynamic aggregation in reactive environments, influenced by thermodynamic and kinetic factors. This dynamic behavior affects catalytic performance, with in situ formed clusters often outperforming single atoms. Operando techniques are essential for understanding structural evolution, while strategies like confinement and defect-engineering regulate aggregation. SACs are thermodynamically unstable, requiring strong metal-support interactions to prevent aggregation. Dynamic aggregation can be reversible or irreversible, impacting activity, selectivity, and stability. Factors such as temperature, voltage, and adsorbates influence this behavior. SACs can switch between active and inactive states under reaction conditions. For example, Rh single atoms on TiO₂ show dynamic coordination changes under different redox conditions. Pt single atoms aggregate in reducing atmospheres but remain stable in oxidizing ones. The stability of SACs is also affected by the support, with reducible oxides like CeO₂ being particularly effective. Multimetallic SACs show unique stability and reactivity due to metal-metal interactions. Dynamic aggregation can enhance catalytic performance, as seen in CO₂ reduction and oxygen reduction reactions. However, it can also lead to catalyst deactivation through sintering or poisoning. Confined SACs in porous materials exhibit distinct dynamic behaviors due to restricted diffusion. Operando techniques like XAS provide insights into the dynamic changes in SACs during reactions. The reversible transformation of SACs into clusters and back again is crucial for maintaining catalytic activity and selectivity. Understanding these dynamics is essential for designing efficient and stable SACs for various catalytic applications.