A mild hybrid system is a means of providing small amounts of electrical assistance to the engine, boosting performance and reducing fuel consumption. Most systems work using an electric motor and generator instead of a conventional starter motor and a small battery pack. Under braking the generator sends electrical energy into the battery pack, which is then used under acceleration to reduce the amount of work the petrol or diesel engine has to do.

Unlike other hybrids they cannot power the vehicle on electricity alone, but they also do not need to be charged externally like plug-in hybrids do. The improvements they offer over conventional petrol and diesel cars are relatively small, but they are also considerably cheaper than plug-in hybrids or full hybrids.