For NO_x control in the presence of excess oxygen, selective catalytic reduction with ammonia (NH₃-SCR) has been successfully applied in the purification of coal-fired flue gas and diesel vehicle exhaust on a large scale. For both cases, various NH₃-SCR catalysts have been developed, with the incorporation of desirable catalytic properties being the central issue. However, the fundamental principle for designing NH₃-SCR catalysts with high activity, selectivity, and stability remain unclear. Generally, in the NH₃-SCR reaction, the redox and acid sites on the catalyst are prerequisites that need to work together. Therefore, the close coupling of these dual functional sites is imperative for the design of NH₃-SCR catalysts with high NO_x removal efficiency. Taking this intrinsic principle into account, we successfully designed and developed various novel catalysts with excellent NH₃-SCR performance. This review will focus on the theory and practice of designing metal oxide catalysts for NH₃ -SCR.