Four-wheel drive, 4WD, or 4x4 ("four by four") is a four-wheeled vehicle with a drivetrain that allows all four wheels to receive torque from the engine simultaneously. In the United States, these cars are often, but not always, included in the broader sport utility vehicle category. While many people associate the term with off-road vehicles, powering all four wheels provides better control in normal road cars on many surfaces, and is an important part of rally racing.Also in some mechanic circles the term "4 by 4" denotes the number of drive wheels and the number of speeds the car has, so the term "5 by 4" would denote an automobile with 5 speeds and 4 drive wheels.

The term four-wheel drive describes truck-like vehicles that require the driver to manually switch between two-wheel drive mode for streets and four-wheel drive mode for low traction conditions such as ice, mud, snow, slippery surfaces, or loose gravel. All-wheel drive (AWD) is often used to describe a "full time" 4WD that may be used on dry pavement without destroying the drivetrain (It should be noted that "Full-Time" 4WD can be disengaged and the center differential can be locked, essentially turning it into regular 4WD. On the other hand, AWD cannot be disengaged and the center differential cannot be locked.) , although the term may be abused when marketing a vehicle. AWD can be used on dry pavement because it employs the use of a center differential, which allows each tire to rotate at a different speed. This eliminates driveline binding, wheel hop and other driveline issues associated with the use of 4WD on dry pavement. With vehicles with more than four wheels, AWD means all wheels drive the vehicle, to varying degrees of engagement while 4WD means only four of the wheels drive the vehicle continuously. Identical drivetrain systems are commonly marketed under different names for upmarket and downmarket branding, and conversely different drivetrain systems are commonly marketed under the same name for brand uniformity. For example, Audi's quattro, DaimlerChrysler's 4Matic used on Mercedes-Benz products, BMW with the xDrive, and Volkswagen's 4motion can mean either an automatically-engaging system with a Haldex clutch or a continuously-operating system with a Torsen differential.

There is often confusion as to the difference between 4X4s and SUVs. This leads to criticisms of 4X4 vehicles in the media that should actually be directed at SUVs (see SUV Criticism).

The Four Wheel Drive Auto Company, more often known as Four Wheel Drive or just FWD, was founded in 1909 in Clintonville, Wisconsin as the Badger Four-Wheel Drive Auto Company by Otto Zachow and William Besserdich.

Zachow and Besserdich developed and built the first successful four-wheel drive (4x4) car, the "Battleship", in 1908. Its success led to the founding of the company. "Badger" was dropped from the name in 1910, and the name was changed to FWD Corporation in 1958.

The success of the four-wheel drive in early military tests prompted the company to switch from cars to trucks. In two world wars, U.S. and Allied armies depended on such four-wheel drive vehicles.

FWD vehicles were made with a track width of 4ft 8½in so they could quickly be used on a standard gauge railway line merely by changing the wheels.

A British subsidiary was set up at Slough in 1921. In 1926, the British FWD, also known as the Quad, was produced with a larger 70bhp engine.

In 1932, AEC took a controlling interest in the British company and began to use more standard AEC components in the Slough-built vehicles. To distinguish these from imported U.S. FWD vehicles, they were marketed under the name Hardy. Production ceased about 1936, but AEC exploited its experience with all-wheel drive in its Second World War Matador (4x4) and Marshall (6x6) vehicles.