Based on number of items held (including musical scores, maps, prints, recordings, etc.), it is the fifth largest library in the United States; based only on volumes (generally books), it is the third largest, after the Library of Congress and Boston Public Library.
Harvard's library system grew from a bequest in 1638 by John Harvard of 400 books.
The location of the library changed over time. Originally it was in the Old College building. In 1676, the library was moved to Harvard Hall, which was where it remained until the building burnt down during the fire in 1764. A new Harvard Hall was built. As time went on space became limited in Harvard Hall, and the library was moved to Gore Hall in 1841. Eventually, Gore Hall was no longer suitable and the books were moved elsewhere in 1912. Around this time, the library spread into more than one building. Some of the libraries were devoted to specialized topics.
On August 1, 2012, a new Harvard Library organization began operations, designed to improve a fragmented system of 73 libraries across Harvard's Schools with one that promotes University-wide collaboration. Functions that occur within all libraries—Access Services, Technical Services and Preservation Services—were unified to enable greater focus on the needs of the user community. The new structure was developed from recommendations of the Task Force on University Libraries and the Library Implementation Working Group.