Selective Catalytic Reduction (SCR) is an advanced active emissions control technology system that reduces tailpipe emissions of nitrogen oxides (NOx) down to near-zero levels in newer generation diesel-powered vehicles and equipment. The system involves several components packaged together with other parts of the emissions control system.

How does SCR work? Hot exhaust gases flow out of the engine and into the SCR system where aqueous urea (known as Diesel Exhaust Fluid, or DEF) is sprayed onto a special catalyst. The DEF sets off a chemical reaction in the exhaust on a special catalyst that converts nitrogen oxides into nitrogen, water, and tiny amounts of carbon dioxide (CO₂), natural components of the air we breathe. The exhaust also passes through a particulate filter somewhere in the system and then is expelled through the vehicle tailpipe.

The design of SCR technology is such that it permits nitrogen oxide (NOx) reduction reactions to take place in an oxidizing atmosphere. It is called "selective" because it reduces levels of NOx using ammonia as a reductant within a catalyst system. The DEF is rapidly broken down to produce the oxidizing ammonia in the exhaust stream.

SCR technology is one of the most cost-effective and fuel-efficient technologies available to help virtually eliminate emissions from diesel engines. Since 2011, all heavy-duty diesel truck engines utilize SCR technology.

SCR is used across construction equipment, agricultural tractors, marine propulsion engines, locomotives, and diesel-powered vehicles. Each manufacturer has its own variations of the type and sequencing of different components in the system to achieve near-zero emissions.