An overview is given of the impact of nitrogen oxides on the environment and human health including the low-temperature selective catalytic reduction of NOx by ammonia (NH3-SCR) over manganese-based materials. The review gives a comprehensive overview of NH3-SCR chemistry including the impacts of NOx, stringent regulations, limitations on NOx emissions and aspects of the reaction mechanism over Mn-based catalysts using isotopic labeled and in situ FT-IR studies. The review attempts to correlate catalyst activity and stability with the acidity, manganese oxidation state, surface texture, and structural morphology. Prospectively, low-temperature SCR of NOx over Mn-based catalysts opens up novel views to mitigate NOx and help us design the next generation industrial catalysts.