A hybrid electric vehicle (HEV) is a type of hybrid vehicle that combines a conventional internal combustion engine (ICE) system with an electric propulsion system. The presence of the electric powertrain is intended to achieve either better fuel economy than a conventional vehicle or better performance. There is a variety of HEV types and the degree to which each functions as an electric vehicle (EV) also varies. The most common form of HEV is the hybrid electric car, although hybrid electric trucks, buses, boats, and aircraft also exist.

A vehicle start-stop system or stop-start system automatically shuts down and restarts the internal combustion engine to reduce the amount of time the engine spends idling, thereby reducing fuel consumption and emissions. This is most advantageous for vehicles which spend significant amounts of time waiting at traffic lights or frequently come to a stop in traffic jams. Start-stop technology may become more common with more stringent government fuel economy and emissions regulations. This feature is present in hybrid electric vehicles, but has also appeared in vehicles which lack a hybrid electric powertrain. For non-electric vehicles fuel economy gains from this technology are typically in the range of 3–10%, potentially as high as 12%. In the United States, idling wastes approximately 14.8 billion liters of gasoline per year.