A glow plug or heater plug is a heating device used to aid starting diesel engines. In cold weather, high speed diesel engines can be difficult to start because the mass of the cylinder block and cylinder head absorb the heat of compression, preventing ignition. Pre-chambered engines use small electric heaters inside the pre-chambers. Direct-injected engines have these glow plugs in the combustion chamber. The glow plug is a pencil-shaped piece of metal with a heating element at the tip.

Some early Perkins engines often used a single heating element in the inlet manifold to provide warm combustion air during the initial cranking of the engine