The Library of Congress (LC) is the research library that officially serves the United States Congress and is the de facto national library of the United States. It is the oldest federal cultural institution in the country. The library is housed in three buildings on Capitol Hill in Washington, D.C.; it also maintains a conservation center in Culpeper, Virginia. The library's functions are overseen by the Librarian of Congress, and its buildings are maintained by the Architect of the Capitol. The Library of Congress is one of the largest libraries in the world. Its "collections are universal, not limited by subject, format, or national boundary, and include research materials from all parts of the world and in more than 450 languages."Congress moved to Washington, D.C., in 1800 after holding sessions for 11 years in the temporary national capitals in New York City and Philadelphia. In both cities, members of the U.S. Congress had access to the sizable collections of the New York Society Library and the Library Company of Philadelphia. The small Congressional Library was housed in the United States Capitol for most of the 19th century until the early 1890s.

Most of the original collection was burnt by the British during the War of 1812, with the library beginning efforts to restore its collection in 1815. The library purchased Thomas Jefferson's entire personal collection of 6,487 books, and its collection slowly expanded in the following years, although it suffered another fire in its Capitol chambers in 1851. This destroyed a large amount of the collection, including many of Jefferson's books. After the American Civil War, the importance of the Library of Congress increased with its growth, and there was a campaign to purchase replacement copies for volumes that had been burned. The library received the right of transference of all copyrighted works to deposit two copies of books, maps, illustrations, and diagrams printed in the United States. It also began to build its collections. Its development culminated between 1888 and 1894 with the construction of its own separate, large library building across the street from the Capitol. Two additional buildings have been constructed nearby to hold collections and provide services, one in the 1930s and one in the 1970s.

The library's primary mission is to research inquiries made by members of Congress, which is carried out through the Congressional Research Service. It also houses and oversees the United States Copyright Office. The library is open to the public for research, although only high-ranking government officials and library employees may check out (i.e., remove from the premises) books and materials.