The Library of Congress is the research library that officially serves the United States Congress, but which is the de facto national library of the United States. It is the oldest federal cultural institution in the United States. John Cole argues that it is now the largest and most international library in the world. He attributes that to its highly influential leaders, especially Ainsworth Rand Spofford (1864–97), Herbert Putnam (1899–1939), Luther H. Evans (1945–53), and James H. Billington (1987–). Cole says they, "have affirmed and expanded Thomas Jefferson's concept that the Library of Congress is a national institution that should be universal in scope and widely and freely available to everyone."
Located in four buildings in Washington, D.C., as well as the Packard Campus in Virginia, it is the second largest library in the world by number of items catalogued. However, such metrics are of limited utility due to the variety of cataloguing methods employed by institutions... Source