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 established by 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 6487 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.

The Library's primary mission is researching inquiries made by members of Congress through the Congressional Research Service. Although it is open to the public, only Members of Congress, Supreme Court justices and other high-ranking government officials may check out books. As the de facto national library, the Library of Congress promotes literacy and American literature through projects such as the American Folklife Center, American Memory, Center for the Book and Poet Laureate.

The Library of Congress was established on April 24, 1800, when President John Adams signed an Act of Congress providing for the transfer of the seat of government from Philadelphia to the new capital city of Washington. Part of the legislation appropriated $5,000 "for the purchase of such books as may be necessary for the use of Congress ..., and for fitting up a suitable apartment for containing them...." Books were ordered from London and the collection, consisting of 740 books and 30 maps, was housed in the new Capitol. Although the collection covered a variety of topics, the bulk of the materials were legal in nature, reflecting Congress' role as a maker of laws.

Thomas Jefferson played an important role in the Library's early formation, signing into law on January 26, 1802 the first law establishing the structure of the Library of Congress. The law established the presidentially appointed post of Librarian of Congress and a Joint Committee on the Library to regulate and oversee the Library, as well as giving the president and vice president the ability to borrow books. The Library of Congress was destroyed in August 1814, when invading British troops set fire to the Capitol building and the small library of 3,000 volumes within.

Within a month, former President Jefferson offered his personal library as a replacement. Jefferson had spent 50 years accumulating a wide variety of books, including ones in foreign languages and volumes of philosophy, science, literature, and other topics not normally viewed as part of a legislative library, such as cookbooks, writing that, "I do not know that it contains any branch of science which Congress would wish to exclude from their collection; there is, in fact, no subject to which a Member of Congress may not have occasion to refer." In January 1815, Congress accepted Jefferson's offer, appropriating $23,950 for his 6,487 books.

The antebellum period was difficult for the Library. During the 1850s the Smithsonian Institution's librarian Charles Coffin Jewett aggressively tried to move that organization towards becoming the United States' national library. His efforts were blocked by the Smithsonian's Secretary Joseph Henry, who advocated a focus on scientific research and publication and favored the Library of Congress' development into the national library. Henry's dismissal of Jewett in July 1854 ended the Smithsonian's attempts to become the national library, and in 1866 Henry transferred the Smithsonian's forty thousand-volume library to the Library of Congress. 

On December 24, 1851 the largest fire in the Library's history destroyed 35,000 books, about two–thirds of the Library's 55,000 book collection, including two–thirds of Jefferson's original transfer. Congress in 1852 quickly appropriated $168,700 to replace the lost books but not for the acquisition of new materials. This marked the start of a conservative period in the Library's administration under Librarian John Silva Meehan and Joint Committee Chairman James A. Pearce, who worked to restrict the Library's activities. In 1857, Congress transferred the Library's public document distribution activities to the Department of the Interior and its international book exchange program to the Department of State. Abraham Lincoln's political appointment of John G. Stephenson as Librarian of Congress in 1861 further weakened the Library; Stephenson's focus was on non-library affairs, including service as a volunteer aide-de-camp at the battles of Chancellorsville and Gettysburg during the American Civil War. By the conclusion of the war, the Library of Congress had a staff of seven for a collection of 80,000 volumes. The centralization of copyright offices into the United States Patent Office in 1859 ended the Library's thirteen year role as a depository of all copyrighted books and pamphlets.

The Library of Congress reasserted itself during the latter half of the 19th century under Librarian Ainsworth Rand Spofford, who directed the Library from 1865 to 1897. Aided by an overall expansion of the federal government and a favorable political climate, Spofford built broad bipartisan support for the Library as a national library and a legislative resource, began comprehensively collecting Americana and American literature, and led the construction of a new building to house the Library, and transformed the Librarian of Congress position into one of strength and independence. Between 1865 and 1870, Congress appropriated funds for the construction of the Thomas Jefferson Building, placed all copyright registration and deposit activities under the Library's control, and restored the Library's international book exchange. The Library also acquired the vast libraries of both the Smithsonian and historian Peter Force, strengthening its scientific and Americana collections significantly. By 1876, the Library of Congress had 300,000 volumes and was tied with Boston Public Library as the nation's largest library. When the Library moved from the Capitol building to its new headquarters in 1897, it had over 840,000 volumes, 40% of which had been acquired through copyright deposit.

A year before the Library's move to its new location, the Joint Library Committee held a session of hearings to assess the condition of the Library and plan for its future growth and possible reorganization. Spofford and six experts sent by the American Library Association, including future Librarian of Congress Herbert Putnam and Melvil Dewey of the New York State Library, testified before the committee that the Library should continue its expansion towards becoming a true national library. Based on the hearings and with the assistance of Senators Justin Morrill of Vermont and Daniel Voorhees of Indiana, Congress more than doubled the Library's staff from 42 to 108 and established new administrative units for all aspects of the Library's collection. Congress also strengthened the office of Librarian of Congress to govern the Library and make staff appointments, as well as requiring Senate approval for presidential appointees to the position.

The Library of Congress, spurred by the 1897 reorganization, began to grow and develop more rapidly. Spofford's successor John Russell Young, though only in office for two years, overhauled the Library's bureaucracy, used his connections as a former diplomat to acquire more materials from around the world, and established the Library's first assistance programs for the blind and physically disabled. Young's successor Herbert Putnam held the office for forty years from 1899 to 1939, entering into the position two years before the Library became the first in the United States to hold one million volumes. Putnam focused his efforts on making the Library more accessible and useful for the public and for other libraries. He instituted the interlibrary loan service, transforming the Library of Congress into what he referred to as a "library of last resort". Putnam also expanded Library access to "scientific investigators and duly qualified individuals" and began publishing primary sources for the benefit of scholars.

Putnam's tenure also saw increasing diversity in the Library's acquisitions. In 1903 he persuaded President Theodore Roosevelt to transfer by executive order the papers of the Founding Fathers from the State Department to the Library of Congress. Putnam expanded foreign acquisitions as well, including the 1904 purchase of a four-thousand volume library of Indica, the 1906 purchase of G. V. Yudin's eighty-thousand volume Russian library, the 1908 Schatz collection of early opera librettos, and the early 1930s purchase of the Russian Imperial Collection, consisting of 2,600 volumes from the library of the Romanov family on a variety of topics. Collections of Hebraica and Chinese and Japanese works were also acquired. Congress even took the initiative to acquire materials for the Library in one occasion, when in 1929 Congressman Ross Collins of Mississippi successfully proposed the $1.5 million purchase of Otto Vollbehr's collection of incunabula, including one of four remaining perfect vellum copies of the Gutenberg Bible.

In 1914 Putnam established the Legislative Reference Service as a separative administrative unit of the Library. Based in the Progressive era's philosophy of science as a problem-solver, and modeled after successful research branches of state legislatures, the LRS would provide informed answers to Congressional research inquiries on almost any topic. In 1965 Congress passed an act allowing the Library of Congress to establish a trust fund board to accept donations and endowments, giving the Library a role as a patron of the arts. The Library received the donations and endowments of prominent individuals such as John D. 

Rockefeller, James B. Wilbur and Archer M. Huntington. Gertrude Clarke Whittall donated five Stradivarius violins to the Library and Elizabeth Sprague Coolidge's donations paid for a concert hall within the Library of Congress building and the establishment of an honorarium for the Music Division. A number of chairs and consultantships were established from the donations, the most well-known of which is the Poet Laureate Consultant.

The Library's expansion eventually filled the Library's Main Building despite shelving expansions in 1910 and 1927, forcing the Library to expand into a new structure. Congress acquired nearby land in 1928 and approved construction of the Annex Building (later the John Adams Building) in 1930. Although delayed during the Depression years, it was completed in 1938 and opened to the public in 1939.

When Putnam retired in 1939, President Franklin D. Roosevelt appointed Archibald MacLeish as his successor. Occupying the post from 1939 to 1944 during the height of World War II, MacLeish became the most visible Librarian of Congress in the Library's history. MacLeish encouraged librarians to oppose totalitarianism on behalf of democracy; dedicated the South Reading Room of the Adams Building to Thomas Jefferson, commissioning artist Ezra Winter to paint four themed murals for the room; and established a "democracy alcove" in the Main Reading Room of the Jefferson Building for important documents such as the Declaration, Constitution and Federalist Papers. Even the Library of Congress assisted during the war effort, ranging from the storage of the Declaration of Independence and the United States Constitution in Fort Knox for safekeeping to researching weather data on the Himalayas for Air Force pilots. MacLeish resigned in 1944 to become Assistant Secretary of State, and President Harry Truman appointed Luther H. Evans as Librarian of Congress. Evans, who served until 1953, expanded the Library's acquisitions, cataloging and bibliographic services as much as the fiscal-minded Congress would allow, but his primary achievement was the creation of Library of Congress Missions around the world. Missions played a variety of roles in the postwar world: the mission in San Francisco assisted participants in the meeting that established the United Nations, the mission in Europe acquired European publications for the Library of Congress and other American libraries, and the mission in Japan aided in the creation of the National Diet Library.

Evans' successor L. Quincy Mumford took over in 1953. Mumford's tenure, lasting until 1974, saw the initiation of the construction of the James Madison Memorial Building, the third Library of Congress building. Mumford directed the Library during a period of increased educational spending, the windfall of which allowed the Library to devote energies towards establishing new acquisition centers abroad, including in Cairo and New Delhi. In 1967 the Library began experimenting with book preservation techniques through a Preservation Office, which grew to become the largest library research and conservation effort in the United States. Mumford's administration also saw the last major public debate about the Library of Congress' role as both a legislative library and a national library. A 1962 memorandum by Douglas Bryant of the Harvard University Library, compiled at the request of Joint Library Committee chairman Claiborne Pell, proposed a number of institutional reforms, including expansion of national activities and services and various organizational changes, all of which would shift the Library more towards its national role over its legislative role. Bryant even suggested possibly changing the name of the Library of Congress, which was rebuked by Mumford as "unspeakable violence to tradition". Debate continued within the library community until the Legislative Reorganization Act of 1970 shifted the Library back towards its legislative roles, placing greater focus on research for Congress and congressional committees and renaming the Legislative Reference Service to the Congressional Research Service.

After Mumford retired in 1974, Gerald Ford appointed Daniel J. Boorstin as Librarian. Boorstin's first challenge was the move to the new Madison Building, which took place between 1980 and 1982. The move released pressures on staff and shelf space, allowing Boorstin to focus on other areas of Library administration such as acquisitions and collections. Taking advantage of steady budgetary growth, from $116 million in 1975 to over $250 million by 1987, Boorstin actively participated in enhancing ties with scholars, authors, publishers, cultural leaders, and the business community. His active and prolific role changed the post of Librarian of Congress so that by the time he retired in 1987, the New York Times called it "perhaps the leading intellectual public position in the nation." Ronald Reagan appointed James H. Billington as the thirteenth Librarian of Congress in 1987, a post he holds as of 2010. Billington took advantage of new technological advancements and the Internet to link the Library to educational institutions around the country in 1991. The end of the Cold War also enabled the Library to develop relationships with newly open Eastern European nations, helping them to establish parliamentary libraries of their own.

In the mid-1990s, under Billington's leadership, the Library of Congress began to pursue the development of what it called a "National Digital Library," part of an overall strategic direction that has been somewhat controversial within the library profession. In late November 2005, the Library announced intentions to launch the World Digital Library, digitally preserving books and other objects from all world cultures. In April 2010, it announced plans to archive all public communication on Twitter, including all communication since Twitter's launch in March 2006.