An ignition coil is used in the ignition system of a spark-ignition engine to transform the battery voltage to the much higher voltages required to operate the spark plug(s). The spark plugs then use this burst of high-voltage electricity to ignite the air-fuel mixture.

The ignition coil is constructed of two sets of coils wound around an iron core. Older engines often use a single ignition coil which has its output directed to each cylinder by a distributor, a design which is still used by various small engines (such as lawnmower engines). Modern car engines often use a distributorless system (such as coil-on-plug), whereby every cylinder has its own ignition coil.

Diesel engines use compression ignition and therefore do not have ignition coils.