The Library of Congress is the research library of the United States Congress, de facto national library of the United States, and the oldest federal cultural institution in the United States. Located in three buildings in Washington, D.C., it is the largest library in the world by shelf space and number of books. The head of the Library is the Librarian of Congress, currently James H. Billington.

The Library of Congress was built for Congress in 1800, and was housed in the United States Capitol for most of the 19th century. After much of the original collection had been destroyed during the War of 1812, Thomas Jefferson sold 6,487 books, his entire personal collection, to the library in 1815. After a period of decline during the mid-19th century the Library of Congress began to grow rapidly in both size and importance after the American Civil War, culminating in the construction of a separate library building and the transference of all copyright deposit holdings to the Library. During the rapid expansion of the 20th century the Library of Congress assumed a preeminent public role, becoming a "library of last resort" and expanding its mission for the benefit of scholars and the American people.