The Army National Guard consists of the "land force" of the United States National Guard, or organized militia, of the several States and Territories, Puerto Rico, and the District of Columbia, active and inactive, as defined in Title 32, USC Section 101. The Army National Guard of the United States is a component of the United States Army. Maintained by the National Guard Bureau, units are under command of the state's governor. An exception to this rule is the D.C Army National Guard; the D.C Army National Guard is under the direction of the President of the United States. When the unit is federalized, the unit falls under the command of the President of the United States.

Like the United States Army Reserve, many members usually serve "One weekend a month, two weeks a year", although many members also serve full-time in order to maintain units, and many units have become more active in military operations in recent years.

The Army National Guard is the oldest branch of the U.S. military, tracing its origins back into American colonies as far back as 1636. English colonists needed to protect themselves and drew civilians into organized militias, from which are descended some units of the Massachusetts Army National Guard. The Army National Guard officially received its title in 1798.

During World War I, the National Guard provided 40% of the US Army's combat forces. Guard membership doubled in 1940, and provided 19 divisions during World War II, of which the National Guard units were among the first to deploy overseas and the first to fight. Later, 138,000 National Guardsmen were deployed for Korea, and many other smaller deployments. During the Vietnam war, almost 23,000 Army and Air Guardsmen were called up for a year of active duty; some 8,700 were deployed to Vietnam. Since September 11, the Army National Guard has been used extensively in Afghanistan and Iraq.