A start-stop system (also referred to as idling stop or micro hybrid) is a technology that automatically shuts down and restarts a vehicle's internal combustion engine to reduce idle time, with the aim of lowering fuel consumption and emissions. The system is most beneficial in urban environments, where vehicles frequently stop and start, such as at traffic lights or in congestion.

Originally developed for hybrid electric vehicles, start-stop systems are now found in a range of conventional vehicles without hybrid powertrains. Reported fuel economy improvements for non-hybrid vehicles range from 3–10%, with some estimates as high as 12%. According to the United States Department of Energy, idling in the United States consumes more than 6 billion U.S. gallons (23 billion liters; 5.0 billion imperial gallons) of fuel annually.

Start-stop operation varies by vehicle type. In manual transmission vehicles, the system typically activates when the gear is in neutral and the clutch is released, and restarts the engine when the clutch is pressed. Automatic systems monitor engine load and accessory demand, and may override stop-start functionality under certain conditions, such as use of air conditioning or low battery charge.

To support engine-off functionality, accessories traditionally powered by a serpentine belt—such as air conditioning compressors and water pumps—may be redesigned to run electrically. Some vehicles, such as the Mazda3 equipped with the i-ELOOP system, use a supercapacitor to temporarily power accessories when the engine is off.

Start-stop technology has also been implemented in two-wheel vehicles, such as Honda scooters sold in Asian and European markets.

Start-stop technology was introduced in Europe first because of regulatory differences: 25 percent of the New European Driving Cycle (NEDC) is spent idling, while only an estimated 11 percent of the United States Environmental Protection Agency (EPA) test is spent idling. Start-stop activation depends on specific driver inputs as well as operating conditions. The engine must have reached the proper temperature to ensure adequate light-off of its catalytic converter and proper lubrication for an effortless restart.

In a car with a manual transmission, engine shutdown typically occurs when braking to a complete stop, shifting the gearbox to neutral, and releasing the clutch. Cars with automatic transmissions shut down upon braking to a full stop; the shutdown is activated by pressing the foot brake pedal when the car comes to a halt. If the car is slowed initially by manual use of the automatic gearbox and final stoppage is achieved by using the handbrake, the engine will not shut down.

A start-stop system only shuts down the engine when it is warm. Most of an engine's wear occurs during cold starts, so the system avoids unnecessary shut downs when the engine is still cold.^{[citation needed]}