In line with many efforts of producing environment-friendly substitutes for fossil-fuel-based compounds (fuels and chemicals), syngas is getting more and more attention. Nowadays, syngas can be produced from various sources via reforming processes. This chapter highlights recent advances in supported metal single-atom catalysts (SACs) devoted to reforming processes, including methane, hydrocarbon, and alcohol reforming. For each process, the basic thermodynamic aspects and main challenges are addressed. Experimental and theoretical calculations reveal the role of the downsizing of active phase from nanoparticles to sub-nanoparticles (clusters and single atoms) in reforming catalysis science. It consists in the lowering of energy barriers to be able to select reaction pathways toward desired syngas products as well as to inhibit/limit carbon deposition.