Officially established in 1903 under the Militia Act, Title 10 and Title 32 of the U.S. Code, the Army National Guard is part of the National Guard of the United States and is divided up into subordinate units stationed in each of the 50 states, three territories and the District of Columbia operating under their respective governors. The Army National Guard may be called up for active duty by the state governors or territorial commanding generals to help respond to domestic emergencies and disasters, such as those caused by hurricanes, floods, and earthquakes.

With the consent of state governors, members or units of the Army National Guard may be appointed, temporarily or indefinitely, to be federally recognized armed force members, in the active or inactive service of the United States. If federally recognized, the member or unit becomes part of the Army National Guard of the United States, which is a reserve component of the United States Army, and part of the National Guard.

Army National Guard units or members may be called up for federal active duty in times of Congressionally sanctioned war or national emergency. The President may also call up members and units of state Army National Guard, with the consent of state governors, to repel invasion, suppress rebellion, or execute federal laws if the United States or any of its states or territories are invaded or is in danger of invasion by a foreign nation, or if there's a rebellion or danger of a rebellion against the authority of the federal government, or if the President is unable with the regular armed forces to execute the laws of the United States. Because both state Army National Guard and the Army National Guard of the United States relatively go hand-in-hand, they are both usually referred to as just Army National Guard.

The term National Guard originally referred to a French citizen militia (see: National Guard (France)). The term is now used in many countries. Depending on the country in question, "national guard" may refer to an organized militia, a military force, a paramilitary force, a gendarmerie, or a police force: