The U.S. Army is one of the three military segments administered by the Department of Defense. The agency is composed of active and reserve components. Its reserve components include the U.S. Army Reserve and the Army National Guard. The U.S. Army is responsible for conducting operational and institutional missions. Its operational section consists of numbered armies, corps, divisions, brigades and battalions that conduct a range of worldwide operations. The agency's institutional organizations provide the infrastructure for training, equipping, deploying and preparing the army for various operations. Its Army Commands include the U.S. Army Forces Command, U.S. Army Material Command, and U.S. Army Training and Doctrine Command. The U.S. Army maintains airfields, barracks, camps, depots, forts and proving grounds. It also operates medical centers, ocean terminals, laboratories and libraries.