An ignition coil, also known as a spark coil, is a component of the ignition system of a spark-ignition engine that transforms 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. The primary winding has relatively few turns of heavy wire, while the secondary winding consists of thousands of turns of smaller wire and is insulated from the high voltage by enamel on the wires and layers of oiled paper insulation. The ignition coil is usually inserted into a metal can or plastic case with insulated terminals for the high voltage and low voltage connections.

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. Most ignition coil systems today use a power transistor to provide pulses to the ignition coil. Modern ignition coils are much smaller because they only need to power one or two spark plugs each. They can be mounted either remotely or directly on top of the spark plug – called coil-on-plug or Direct Ignition.

The symptoms of a failing ignition coil include misfires, stalling, backfiring, engine misfiring, and difficulty starting the engine. A failed ignition coil can severely damage the engine, so it is important to keep an eye out for these symptoms.