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For years, secondary air injection Systems have been used to reduce hydrocarbon exhaust emissions for a short period after engine cold start. In the beginning, passive secondary air systems were used, with the airflow driven by the pressure pulsations in the exhaust system. Since 1990, for most applications, active secondary air systems (i. e., systems where air is injected into the hot exhaust gases by a pump) have been employed. Secondary air injection into the hot exhaust gases is realized by a d-c motor driven turbine pump, i. e. a secondary air pump, and a control valve. Numerous factors, including raw engine emissions during cold start and warm up, driveability requirements and the need to adapt to different emissions legislation, dictate the use of secondary air injection systems. The development of other exhaust aftertreatment systems, e. g., close-coupled or heated catalysts as well as packaging and cost factors will influence the market penetration of secondary air systems.