Selective catalytic reduction of NOx using NH3 or hydrocarbons (NH3-SCR or HC-SCR) in oxygen-rich exhaust from diesel engines remains a major challenge in environmental catalysis. The development of highly efficient, stable and environmentally-benign catalysts for SCR processes is very important for practical use. In this feature article, the structure-activity relationship of vanadium-free catalysts in the NH3-SCR reaction is discussed in detail, including Fe-, Ce-based oxide catalysts and Fe-, Cu-based zeolite catalysts, which is beneficial for catalyst redesign and activity improvement. Based on our research, a comprehensive mechanism contributing to the performance of Ag/Al2O3 in HC-SCR is provided, giving a clue to the design of a catalytic system with high efficiency.