Understanding the reaction mechanisms of CO₂ and CO methanation processes is critical towards the successful development of heterogeneous catalysts with better activity, selectivity, and stability. This review provides detailed mechanisms of methanation processes and undesired catalyst deactivation. We characterize the methanation processes into two categories: (1) associative scheme, in which hydrogen atoms are involved in the C–O bond breaking process, and (2) dissociative scheme, where C–O bond breaking takes place before hydrogenation. For the catalyst deactivation mechanisms, we highlight three important factors, i.e. sulfur poisoning, carbon deposition and metal sintering.