The Selected Reserve is the most readily available group of Army Reserve Soldiers to the President. The Selected Reserve is comprised of Troop Program Units (TPUs), Active Guard and Reserve (AGR) Soldiers and Individual Mobilization Augmentees (IMAs). In the event of an emergency, all members of the Selected Reserve and up to 30,000 members of the Individual Ready Reserve (IRR) can be mobilized.

The Army Reserve is authorized to have more than 185,000 Soldiers in more than 6,000 Troop Program Units (TPU). These men and women typically train on selected weekends and perform annual training.

AGR Soldiers serve full time in units and organizations of the Army Reserve or that directly support the Army Reserve.

The Army Reserve's IMAs are assigned to high-level headquarters where they would serve if mobilized. Most IMAs train annually for two weeks.

Members of the Individual Ready Reserve (IRR) are trained Soldiers who may be called upon, if needed, to replace Active Duty and Army Reserve Soldiers. Many of the Soldiers in the IRR have left the military recently and still have an Army Reserve commitment. Others have chosen to remain as Army Reserve Soldiers, but not as a unit member or IMA. In all, there are about 163,000 members of the IRR.

The Retired Reserve consists of approximately 715,000 retirees from the Army (Army, Army Reserve and Army National Guard) who remain part of the Army Reserve family.