A headlight or headlamp is a light, usually attached to the front of a vehicle such as a car, with the purpose of illuminating the road ahead during periods of low visibility , such as night or precipitation.

Modern headlights are electric floodlights, positioned in pairs, one on each side of a moving vehicle. Headlights are usually combinations of multiple lamps and reflectors and can be switched between two modes or configurations: low beams and high beams. High beams cast more light at a higher angle, allowing the driver to see further away, but at the cost of potentially blinding drivers of oncoming vehicles. High beams also increase reflection from fog, due to the refraction of the water droplets.

In the USA, many headlights are of the sealed beam type, meaning that the reflector, lens array on the front, as well as bulb, are all one unit that must be replaced together. In most European countries sealed beam headlights are not allowed on automobiles. 'H4' halogen incandescent light bulbs are used instead.

Several countries, including Canada and the Scandinavian countries, require daytime running lights (DRLs) or the use of low-beam headlights during daytime driving as well. Most often, this is the high beams running at half power. Many models of automobiles sold elsewhere are also equipped with daytime running lights, which are automatic when the car is running. This is entirely for the visibility of the car itself by other drivers, especially when there are low-light conditions which a driver may forget to turn on his or her regular headlights. A slight disadvantage of the DRL is that it wastes a small amount of gasoline or petrol, especially with incandescent light bulbs.

Most headlights use incandescent light bulbs (usually halogen-type), either with separate high and low beam bulbs, or a single bulb on each side with dual filaments. More and more are using high-intensity discharge (HID) lamps, similar to the Mercury-vapor lamps used in white street lights. These have a distinct purplish or bluish cast to them, which also causes fluorescence in certain materials. HID bulbs are also extremely energy-efficient, using over 80% less power, and drawing less electrical current from the alternator and improving fuel efficiency.

A headlight can also be mounted on a bicycle (with a battery or small generator), and most other moving vehicles from airplanes to trains tend to have headlights of their own. Single small headlights may also be mounted on a helmet designed to be worn in situations where light is required but both hands are needed, for example in subterranean mines or for spelunking in caves.

Headlights usually have a distinctive shape with a light bulb positioned in the focus of a parabolic reflector cone. The glass lens usually has additional patterns to direct the lightbeam in a certain direction. These patterns are different (mirrored) for left- or right hand traffic.

Headlights must be adjusted, aimed to an ISO-regulated inclination and center. This applies most of all to low beams (US name) or dipped beams (UK name), where the aiming results in a specific position of the cut-off line in the beam when projected on a wall at a minimum 10-metre (33-foot) distance from the car. The cut-off line is the transition between the illuminated and dark section of the beam. Headlights are checked for alignment prior to leaving the production line, and may need realignment periodically throughout the car's lifetime, or after repairing body damage.

Directional headlamp

These provide improved lighting for cornering. Some automobiles have their headlights connected to the steering mechanism so the lights will follow the movement of the front wheels. The Citroën DS was one car equipped with such a system. Also, some automobiles have vertically adjustable lights, to compensate for dipping when carrying heavy loads.

Dual-beam headlamps

Night driving has long been dangerous due to the glare of headlights that blind drivers approaching from the opposite direction. Therefore, headlights that satisfactorily illuminate the highway ahead of the automobile for night driving without temporarily blinding approaching drivers have long been sought. To correct this problem resistance-type dimming circuits, which decreased the brightness of the headlights when meeting another car, were first introduced. This gave way to mechanical tilting reflectors and later to double-filament bulbs with a high and a low beam, called sealed-beam units.

There was only one filament at the focal point of the reflector in the double-filament headlight unit of necessity. Greater illumination required for high-speed driving with the high beam, consequently, the lower beam filament was placed off center, with a resulting decrease in lighting effectiveness. From the 1950s, manufacturers equipped their models with four headlights to improve illumination.

In some cars, dimming is automatically achieved. This happens by means of a photocell-controlled switch in the lamp circuit that is triggered by the lights of an oncoming car. Larger double-filament lamps and halogen-filled lamp bulbs with improved photometrics permitted a return to two-headlight systems on some cars. At many places the law limits the total intensity of forward lighting systems to 75,000 candlepower (800,000 lux).

In most new automobiles, lowering front hood heights for improved aerodynamic drag and driver visibility reduces the vertical height available for headlights. Due to this, lower-profile rectangular sealed-beam units and higher-intensity bulbs, in conjunction with partial parabolic reflectors with reduced vertical axis, were adopted in the 1970s. In some cases, models featured full-size concealed headlights that were not visible until turned on. An electric motor linkage was used to rotate the lamp housing or a housing cover into proper position to supply lighting. Aerodynamic benefits were provided by this system only when the headlights were concealed.

In the 1960s, signal lamps and other special-purpose lights were increased in usage. Amber-colored front and red rear signal lights are flashed as a turn indication; all these lights are flashed simultaneously in the "flasher" system for use when a car is parked along a roadway or is traveling at a low speed on a high-speed highway. The law requires that marker lights that are visible from the front, side, and rear be also present. Red-colored rear signals are used to denote braking, and, on some models, cornering lamps to provide extra illumination in the direction of an intended turn are available. These are actuated in conjunction with the turn signals. To provide illumination to the rear when backing up, backup lights are required.