Fuel injection is a system of fuel delivery for mixture with air in an internal combustion engine. It has become the primary system used in automotive engines, having replaced carburetors primarily in the 1980s.

A fuel injection system is designed and calibrated specifically for the type(s) of fuel it will handle: gasoline (petrol), Autogas (LPG, also known as propane), ethanol, methanol, methane (natural gas), hydrogen or diesel. The majority of fuel injection systems are for gasoline or diesel applications. With the advent of electronic fuel injection (EFI), the diesel and gasoline hardware has become similar. EFI's programmable firmware has permitted common hardware to be used with multiple different fuels. For gasoline engines, carburetors were the predominant method to meter fuel before the widespread use of fuel injection. However, a wide variety of injection systems have existed since the earliest usage of the internal combustion engine.

The primary functional difference between carburetors and fuel injection is that fuel injection atomizes the fuel by forcibly pumping it through a small nozzle under high pressure, while a carburetor relies on the vacuum created by intake air rushing through it to add the fuel to the airstream.

The fuel injector is only a nozzle and a valve: the power to inject the fuel comes from farther back in the fuel supply, from a pump or a pressure container.