The United States Army is made up of three components: the active component, the Regular Army; and two army reserve components, the Army National Guard and the US Army Reserve. Both reserve components are primarily composed of part-time soldiers who train once a month, known as Battle Assembly or Unit Training Assemblies (UTAs), and conduct two to three weeks of annual training each year. Both the Regular Army and the Army Reserve are organized under Title 10 of the United States Code, while the National Guard is organized under Title 32. While the Army National Guard is organized, trained and equipped as a component of the U.S. Army, when it is not in federal service it is under the command of individual state and territorial governors; the District of Colubmia National Guard, however, reports to the U.S. President, not the District's Mayor, even when not federalized. Any or all of the National Guard can be federalized by presidential order and against the governor's wishes.

The Army is led by a civilian Secretary of the Army, who has the statutory authority to conduct all the affairs of the Army; under the authority, direction and control of the Secretary of Defense. The Chief of Staff of the Army who is the highest ranked military officer in the Army has dual roles; one as the principal military adviser and executive agent for the Secretary of the Army, i.e. its service chief; and secondly as a member of the Joint Chiefs of Staff, a body composed of the service chiefs from each of the four military services belonging to the Department of Defense who advise the President of the United States, the Secretary of Defense, and the National Security Council on operational military matters, under the guidance of the Chairman and Vice Chairman of the Joint Chiefs of Staff. In 1986, the Goldwater-Nichols Act mandated that operational control of the services follows a chain of command from the President to the Secretary of Defense directly to the Unified Combatant Commanders, who have control of all armed forces units in their geographic or function area of responsibility. Thus, the Secretaries of the military departments (and their respective service chiefs underneath them) only have the responsibility to organize, train and equip their service components. The Army provides trained forces to the Combatant Commanders for use as directed by the Secretary of Defense.

Through 2013, the Army is shifting to six geographical commands that will line up with the six geographical Unified Combatant Commands (COCOM):

• United States Army Central headquartered at Fort McPherson, Georgia
• United States Army North headquartered at Fort Sam Houston, Texas
• United States Army South headquartered at Fort Sam Houston, Texas
• United States Army Europe headquartered at Campbell Barracks, Germany
• United States Army Pacific headquartered at Fort Shafter, Hawaii (eventually to be merged with the Eighth Army).
• United States Army Africa headquartered at Vicenza, Italy

The Army is also changing its base unit from divisions to brigades. When finished, the active army will have increased its combat brigades from 33 to 48, with similar increases in the National Guard and Reserve forces. Division lineage will be retained, but the divisional HQs will be able to command any brigades, not just brigades that carry their divisional lineage. The central part of this plan is that each brigade will be modular, i.e. all brigades of the same type will be exactly the same, and thus any brigade can be commanded by any division. There will be three major types of ground combat brigades:

• Heavy brigades will have around 3,700 troops and be equivalent to a mechanized infantry or tank brigade.
• Stryker brigades will have around 3,900 troops and be based on the Stryker family of vehicles.
• Infantry brigades will have around 3,300 troops and be equivalent to a light infantry or airborne brigade.

In addition, there are combat support and service support modular brigades. Combat support brigades include Aviation brigades, which will come in heavy and light varieties, Fires (artillery) brigades, and Battlefield Surveillance brigades. Combat service support brigades include Sustainment brigades and come in several varieties and serve the standard support role in an army.