The National Guard is a state-based military force that becomes part of the U.S. military's reserve components of the U.S. Army and the U.S. Air Force when activated for federal missions. It is a military reserve force composed of National Guard military members or units of each state and the territories of Guam, the Virgin Islands, Puerto Rico, and the District of Columbia, for a total of 54 separate organizations. It is officially created under Congress's Article 1 Section 8 ability to "raise and support armies". All members of the National Guard are also members of the organized militia of the United States as defined by 10 U.S.C. § 246. National Guard units are under the dual control of state governments and the federal government.

The majority of National Guard soldiers and airmen hold a civilian job full-time while serving part-time as a National Guard member. These part-time guardsmen are augmented by a full-time cadre of Active Guard & Reserve (AGR) personnel in both the Army National Guard and Air National Guard, plus Army Reserve Technicians in the Army National Guard and Air Reserve Technicians (ART) in the Air National Guard.

The National Guard is a joint activity of the United States Department of Defense (DoD) composed of reserve components of the United States Army and the United States Air Force: the Army National Guard and the Air National Guard, respectively.

Colonial militias were formed during the British colonization of the Americas from the 17th century onward. The first colony-wide militia was formed by Massachusetts in 1636 by merging small, older local units, and several National Guard units can be traced back to this militia. The various colonial militias became state militias when the United States became independent. The title "National Guard" was used in 1824 by some New York State militia units, named after the French National Guard in honor of the Marquis de Lafayette. "National Guard" became a standard nationwide militia title in 1903, and has specifically indicated reserve forces under mixed state and federal control since 1933.