Hydrogen is a clean fuel widely used in fuel cells, engines, rockets and many other devices. The catalytic decomposition of methane (CDM) is a COx-free hydrogen production technology from which carbon nano materials (CNMs) can be generated as a high value-added byproduct for electrode, membranes and sensors. Recent work has focused on developing a low cost catalyst that could work without rapid deactivation by carbon deposition. In this review, the economic and environmental evaluation of CDM are compared with coal gasification, steam reforming of methane, and methanol steam reforming in terms of productivity, CO2 emissions, and H2 production and cost. CDM could be a favorable technology for on-site demand-driven hydrogen production on a small or medium industrial scale. This study covers the Fe-based, Ni-based, noble metal, and carbonaceous catalysts for the CDM process. Focusing on hydrogen (or carbon) yield and production cost, Fe-based catalysts are preferable for CDM. Although Ni-based catalysts showed a much higher hydrogen yield with 0.39 molH2/gcat./h than Fe-based catalysts with 0.22 molH2/gcat./h, the hydrogen cost of the former was estimated to be 100-fold higher ($0.89/$0.009). Further, the CDM performance on different types of reactors are detailed, whereas the molten-metal catalyst/reactor is suggested to be a promising route to commercialize CDM. Finally, the formation mechanism, characterization, and utilization of carbon byproducts with different morphologies and structures are described and analyzed. Versus other reviews, this review shows that cheap Fe-based catalysts (10 tons H2/1 ton iron ore) and novel molten-metal reactors (95% methane conversion) for CDM are feasible research directions for a fundamental understanding of CDM. The CNMs by CDM could be applied to the waste water purification, lubricating oils, and supercapacitors.