The production of methane from the reaction between CO₂ and H₂ (CO₂ methanation) has gained increasing attention in recent years. The rational design of novel catalytic materials for this reaction will depend on the fundamental description of the active sites and the identification of surface reaction intermediates. Currently, there is a debate regarding the mechanism for the CO₂ methanation on supported metals, with apparently contradictory proposals suggesting that various surface species could be either reaction intermediates or spectators. Similarly, there is a discussion regarding the nature of the surface sites on the catalysts that activate the CO₂ molecule during the reaction. Specifically, it has been suggested that different reaction routes could occur on different metalsupport combinations and on various surface structures. In this manuscript, we critically review the literature on CO₂ methanation and discuss the physical evidence that has been presented to propose reaction mechanisms on various supported metals. The relevance of the presence of nanosized metal particles in the catalysts is also discussed.