The Pentagon is the headquarters of the United States Department of Defense, located in Arlington County, Virginia, across the Potomac River from Washington, D.C. As a symbol of the U.S. military, The Pentagon is often used metonymically to refer to the U.S. Department of Defense. The Pentagon's three occupant groups, military, civilians, and contractors, contribute to the planning and execution of the defense of our country.

Headquarters of the Department of Defense, the Pentagon is one of the world's largest office buildings. Built in just 16 months, the Pentagon is the world's largest low-rise office building. It is twice the size of the Merchandise Mart in Chicago; it has three times the floor space of the Empire State Building in New York; and the National Capital could fit into any one of the five wedge-shaped sections. The Pentagon is virtually a city in itself. In terms of population, the Pentagon's 25,650 employees make it larger than nine out of 10 towns in the United States.

Employees, both military and civilian people arrive daily from Washington, D.C. and its suburbs over approximately 30 miles of access highways, including express bus lanes and one of the newest subway systems in our country. These people make over 200,000 telephone calls daily through phones connected by 100,000 miles of telephone cable.

The Defense Post Office handles about 1,200,000 pieces of mail monthly.
Various libraries support staff in research and completion of their work. The Army Library provides 300,000 publications and 1,700 periodicals in various languages.

The Pentagon, originally known as "Arlington Farms," began construction on September 11th, 1941 (exactly 60 years prior to the 9/11 attacks) on a plot of land which was once part of the grand estate of the Confederate general Robert E. Lee, before its confiscation during the Civil War.

The Pentagon was conceived at the request of Brigadier General Brehon B. Sommervell, Chief of the Construction Division of the Office of the Quartermaster General. The original purpose of the Pentagon was to provide a temporary solution to the War Department's critical shortage of space. Post WWII plans were to recycle the Pentagon into hospital, office, or warehouse space.

The site designated for construction was bordered by five roadways which dictated the concept for a Pentagon-shaped building. In addition to the 296 acres the Federal government owned, an additional 287 acres (including the old Hoover Airport and a low income residential area called "Hell's Bottom") were purchased. The total cost to develop the Pentagon Reservation was over $83 million, which included about $2.25 million to purchase land, $33.40 million to construct outside facilities and the Pentagon Lagoon, and $49.60 million to construct the Pentagon. The 16 month project required the skill of 1,000 architects and the labor of 14,000 tradesmen (on three shifts, 24 hours a day). At the height of WWII, the Pentagon housed over 33,000 occupants.