SCR (Selective Catalytic Reduction) is a system in which reductant is added to the exhaust gases in order to reduce the nitrogen oxide (NOx) content.

Reductant is a solution consisting of urea and water, and is usually called AdBlue®, DEF, ARLA 32 eller AUS 32, depending on the market. If the engine is equipped with an SCR system, the reductant is added to the exhaust gases upstream of the catalytic converter. This reduces nitrogen oxide emissions.