A block heater is used in cold climates to warm an engine prior to starting. They are mostly used for car engines; however, they have also been used in aircraft engines.

The most common design of block heater is an electrical heating element embedded in the engine block.

Pre-heating of an engine is primarily used to make it easier to start. Added benefits are:

Block heaters or coolant heaters are also found on the diesel engines in standby generators, to reduce the time taken for the generator to reach full power output in an emergency.

The vehicle operator must have access to electrical power. In cold climates, parking areas for residents, employees, or students, or paid public parking areas, may have electrical outlets.

The operator must disconnect the power cord before driving away. Forgetting to disconnect can result in damage to the power cord or the vehicle. After a snow storm, the cable may not be visible under the snow. Residential overnight street parking may not allow an extension cord to cross a public sidewalk.

The energy used to heat the engine adds a cost. However, the savings in fuel generally outweigh this cost, especially if a timer is used to limit the heating period to about 4 hours before the expected start time. Taking the needed precautions, a kerosene jet-heater can also be used to pre-heat the engine.

Some cars are produced with block heaters from the factory, while others are fitted with block heaters as an aftermarket add-on. The most common type of block heater is an electric heating element in the engine block, which is connected to an AC power cord often routed through the vehicle's grille. Some block heaters are designed to replace one of the engine's core plugs. Both heat the engine via the coolant.