Although the concept of isolated atoms featuring unique reactivity has long existed in homogenous and enzymatic catalysis, the field of metal single-atom heterogenous catalysts is only recently establishing its unique position in bridging homogenous and heterogenous catalysis, by bringing benefits from both fields. Metal single-atom catalysts have been applied in a wide range of chemical transformations, which include oxidations, hydrogenations, carbon-carbon and carbon-heteroatom couplings, hydroelementation, carboxylation, fluorination, cycloaddition reactions, oxygen or carbon dioxide or nitrogen reduction reactions, and oxygen or hydrogen evolution reactions. In the context of electro-and photo-catalytic hydrogenations, copper single-atom catalysts supported either on titania or nitrogen-doped carbon were shown to be active in the acetylene electroreduction, palladium or nickel single-atom catalysts supported on carbon nitride, were shown to work for the photocatalytic hydrogenation of substituted alkenes and alkynes (e.g., styrene, 2ethynylnaphthalene) in water or water/organic solvent mixture.