From the mid-1970s and on, the contribution to air pollution of carbon monoxide, hydrocarbons and nitrogen oxides from gasoline passenger cars in the developed world has been reduced through co-evolution of regulation and commercial introduction of catalytic emission control technology, now part of hundreds of millions of cars, trucks and buses worldwide. This dissertation is a disaggregated study of the global introduction of catalytic emission control technology as a measure to reduce local air pollution. The introduction of the "three-way" catalyst for gasoline passenger cars is studied for four countries. Present innovation in diesel engine emission control is studied. Technological change is analyzed regarding the process of innovation, the innovation system and its stakeholders.