Tank, an armored motor vehicle designed to fight while it is moving. It differs from other armored vehicles—such as self-propelled guns and personnel carriers—in that its primary mission is to attack, rather than to defend, transport, or scout. The branch of the U.S. Army concerned with tank warfare is called Armor. The tank is the principal weapon of armored divisions and an important supporting weapon of infantry divisions. In the U.S. Army, an armored division has about 300 tanks, and an infantry division about 50.

Tanks move on crawler, or Caterpillar-type, tracks, permitting them to travel cross country. The engine—a gasoline or diesel internal-combustion engine or a turbine engine—is mounted in the rear and transmits power to the tracks by geared wheels. Other wheels, variously called bogies, idlers, or road wheels, support the tank on its tracks but do not transmit power. Most present-day tanks have maximum speeds of 30 to 45 mph (48 to 72 km/h) and can travel 130 to about 300 miles (210 to 480 km) without refueling.

The main gun of a modern tank ranges in size (bore diameter) from 75 to 125 mm and is usually mounted in a turret that can be turned in a full circle. The effective range of the gun varies with the type of ammunition and the target. A high-velocity 105-mm gun can be effective against a tank up to 3,500 yards (3,200 m) away and against troops at a distance of about 8,000 yards (7,300 m). A gyroscopic stabilizing device steadies the gun to keep it on target when the tank is moving over rough ground. Tanks also carry machine guns.

The typical tank has a crew of four—a commander (usually a sergeant or lieutenant), a driver, a gunner, and a loader. The driver looks through a periscope and steers the tank with controls that vary the speed of the tracks. (In a right turn, for example, the left track moves faster than the right track.) A two-way radio links each tank with other members of its unit.

There are two basic kinds of tanks, the main battle tank (MBT) and the light tank. The MBT is heavily armored and weighs from 32 to 45 metric tons. The caliber of its main gun ranges from 105 to 125 mm. The MBT is primarily an offensive weapon, designed to break through enemy defenses for such purposes as opening the way for infantry, cutting the enemy's supply lines, or dashing ahead to capture strategic points. Even in a defensive situation, the MBT, where possible, is employed offensively, meeting enemy tanks with attacks of its own.

The light tank is less heavily armored and usually less well-armed, carrying a 75-mm or 90-mm gun. It is designed for reconnaissance, but sometimes is outfitted with a 105-mm gun to perform as a low-cost substitute for the MBT. Because of its relatively light weight, about 14 metric tons, the light tank can be easily transported in airplanes for use by airborne assault forces. Light tanks are also used in counterinsurgency warfare and in quelling civil disturbances.

All tanks have certain limitations. They are handicapped in mountainous or swampy regions. Increases in armor protection and in weight of armament cut down speed and cruising range. Maintenance of the complicated machinery is a constant problem, as is supply, especially of fuel and ammunition.