Single-atom catalysts (SACs) have emerged as a focal point of research in the field of heterogeneous catalysis. This paper reviews the progress in the studies of single atoms as promoters in various catalytic reactions, elucidating their distinctive role in comparison to the dominant active sites. We provide a discussion on the application of single-atom promoters (SAP) within host-guest systems in various catalysts, including metal oxide supported catalysts, molybdenum carbide-based catalysts, bimetallic catalysts, and others. The behavior of SAP is diverse. They often promote the formation of oxygen vacancies for oxide support, leading to local site reconstruction that creates specific reaction route. Moreover, they can also precisely modify the electronic structure of hetero-metal atomic or nanoparticle sites, then regulating the adsorption of reactants or intermediates and catalytic performance. Finally, the potential for the development of SAP is outlined, proposing novel approach for the design of SACs with enhanced activity and stability.