This review discusses the recent advances in synergistic electrocatalysis between single atoms and nanoparticles/clusters, focusing on the design and performance of composite catalysts. The synergistic effect between various active sites enhances electrocatalytic performance, while high atomic utilization provides insights into the relationship between structure and activity. The review highlights the challenges and future directions for multi-active site catalysts. Single-atom catalysts (SACs) offer high efficiency and activity per atom, but their performance is limited by their simple structure and lack of synergistic active sites. Integrating SACs with clusters and nanoparticles can overcome these limitations by enhancing catalytic activity, longevity, and reaction dynamics. The review summarizes the synthesis strategies, characterization methods, and key factors governing the structure of these composite catalysts. It also discusses various clean energy catalytic reactions, such as oxygen reduction/evolution reaction (ORR/OER), hydrogen evolution reaction (HER), and carbon dioxide reduction reaction (CO₂RR), performed over synergistic composite catalysts. The review emphasizes the importance of understanding the synergistic mechanism between single atoms and clusters/nanoparticles, including the interaction between them at the atomic level. Density functional theory (DFT) calculations are used to analyze the electronic structure and reaction mechanisms. The review also discusses the role of different types of synergistic components, such as single atomic site-mono-metal nanoparticle, single atomic site-alloy nanoparticle, single atomic site-transition metal compounds (TMCs) nanoparticles, and single atomic site-clusters. These components can enhance catalytic performance by modulating the electronic structure, improving the adsorption of intermediates, and promoting reaction pathways. The review concludes that integrating single atoms with clusters and nanoparticles is an effective way to enhance electrocatalytic performance, reduce the use of precious metals, and improve the stability and durability of catalysts. The synergistic effect between these components provides new perspectives for the design of rational catalysts and elucidates their optimal mechanisms in reactions.This review discusses the recent advances in synergistic electrocatalysis between single atoms and nanoparticles/clusters, focusing on the design and performance of composite catalysts. The synergistic effect between various active sites enhances electrocatalytic performance, while high atomic utilization provides insights into the relationship between structure and activity. The review highlights the challenges and future directions for multi-active site catalysts. Single-atom catalysts (SACs) offer high efficiency and activity per atom, but their performance is limited by their simple structure and lack of synergistic active sites. Integrating SACs with clusters and nanoparticles can overcome these limitations by enhancing catalytic activity, longevity, and reaction dynamics. The review summarizes the synthesis strategies, characterization methods, and key factors governing the structure of these composite catalysts. It also discusses various clean energy catalytic reactions, such as oxygen reduction/evolution reaction (ORR/OER), hydrogen evolution reaction (HER), and carbon dioxide reduction reaction (CO₂RR), performed over synergistic composite catalysts. The review emphasizes the importance of understanding the synergistic mechanism between single atoms and clusters/nanoparticles, including the interaction between them at the atomic level. Density functional theory (DFT) calculations are used to analyze the electronic structure and reaction mechanisms. The review also discusses the role of different types of synergistic components, such as single atomic site-mono-metal nanoparticle, single atomic site-alloy nanoparticle, single atomic site-transition metal compounds (TMCs) nanoparticles, and single atomic site-clusters. These components can enhance catalytic performance by modulating the electronic structure, improving the adsorption of intermediates, and promoting reaction pathways. The review concludes that integrating single atoms with clusters and nanoparticles is an effective way to enhance electrocatalytic performance, reduce the use of precious metals, and improve the stability and durability of catalysts. The synergistic effect between these components provides new perspectives for the design of rational catalysts and elucidates their optimal mechanisms in reactions.