the increasingly serious energy crisis, environmental challenges with clean and sustainable energy conversion. Hydrogen (H 2 ) has attracted great attention because of its cleanliness and extremely high weight energy density (122 kJ g −1 ). [1] As we all know, the final product from H 2 energy is water no matter how it is used, which can truly achieve zero-emission and zero-pollution. Therefore, H 2 energy is considered to be one of the most ideal energy sources in the future. [2] Traditional H 2 production technologies are mainly formed by steam reforming of fossil fuels, such as natural gas and petroleum. [3] The key point that must be paid attention to is to reduce pollution and improve the efficiency of H 2 evolution from fossil fuels. In the future, H 2 production industrially should present the coexistence form of diversified development in which the advantages of the fossil raw material route and the renewable raw material route complement each other.Water electrolysis and photocatalytic water splitting have been promising techniques for sustainable H 2 production, and they are important supplements to the H 2 generation from fossil fuels. However, there are still plenty of technical problems industrially, which hinder their wide application in practice. On the one hand, with the continuous increase in the global energy demand, noble metal-based catalysts dominated by platinum group metal (PGM) have severely restricted the further development of the energy industry owing to their high cost, natural scarcity, and poor anti-poisoning. On the other hand, the catalytic efficiency of earth-abundant transition-metal-based nanocatalysts employed as an alternative to PGM-based catalysts is far from the criterion of industrial applications. [4] It can be seen that improving the utilization of noble metal active centers and developing efficient noble metal-free catalysts are available solutions. This is still a hugely challenging task, for which we should first survey the reasonable development of the catalytic system.The evaluation indexes of the catalytic reaction include selectivity and conversion. The key points that need to be studied are: i) active center and adsorption site; ii) the size-dependent effects of catalysts; iii) interactions between active species and the support, surface effect, and interfacial effect. The Hydrogen is widely believed to be a promising fuel to solve the global energy crisis and environmental issues. The catalytic system represented by metal-supported catalysts is an important process of upgrading the hydrogen source in industry. Single-atom catalysts (SACs), which inherit the advantages of homogeneous and heterogeneous catalysts, provide a broad prospect for low-cost H 2 production technology. This review focuses on the potential mechanisms in the rational design of SACs, including active sites, coordination configuration, mass loading, heteroatom-doping, and metal−support interaction. The design strategies of single metal atoms on different supports are reviewed to give a propos...