The Pentagon is the headquarters of the United States Department of Defense, located in Arlington, Virginia. As a symbol of the U.S. military, "the Pentagon" is often used metonymically to refer to the Department of Defense rather than the building itself.

Designed by architect George Bergstrom (1876 - 1955) and built by Philadelphia, Pennsylvania general contractor John McShain, the building was dedicated on January 15, 1943 after ground was broken for construction on September 11 1941. Colonel Leslie Groves was responsible for overseeing the project in 1940-42, and his administrative ability, drive and forcefulness led to his nomination to head the Manhattan Project in 1942.

The Pentagon is one of the world's largest office buildings. blank">The Pentagon, Facts & Figures (accessed January 19, 2008) It houses approximately 23,000 military and civilian employees and about 3,000 non-defense support personnel. It has five sides, five floors above ground (plus two basement levels), and five ring corridors per floor with a total of 17.5 miles (28.2 kilometers) of corridors.

At five acres (20,000 m²), the central plaza in the Pentagon is the world's largest "no-salute, no-cover" area (an area exempt from the normal rule that, when out of doors, U.S. military personnel must wear hats and salute superior officers) outside of a combat zone. The open space in the center is informally known as "ground zero," a nickname originating during the _Cold War and based on the presumption that the Soviet Union would target one or more nuclear missiles at this central location. At the center of this plaza is the "Ground Zero Cafe," a snack bar.

Just south of the Pentagon are Pentagon City and Crystal City, extensive shopping and high-density residential districts in Arlington. Arlington National Cemetery is to the north. The Washington Metro Pentagon station is also located at the Pentagon, on the Blue and Yellow Lines. The Pentagon is surrounded by the complex Pentagon road network.