The British Library is the national library of the United Kingdom. Based in London, it is the largest library in the world, with an estimated collection of over 200 million items from multiple countries. As a legal deposit library, it receives copies of all books produced in the United Kingdom and Ireland, as well as a significant proportion of overseas titles distributed in the United Kingdom. The library operates as a non-departmental public body sponsored by the Department for Culture, Media and Sport.

The British Library is a major research library, with items in many languages and in many formats, both print and digital: books, manuscripts, journals, newspapers, magazines, sound and music recordings, videos, play-scripts, patents, databases, maps, stamps, prints, drawings. The Library's collections include around 14 million books, along with substantial holdings of manuscripts and items dating as far back as 2000 BC. The library maintains a programme for content acquisition and adds some three million items each year occupying 9.6 kilometres (6 mi) of new shelf space.

The Library's purpose-built building stands next to St Pancras station in London. It was officially opened by Elizabeth II on 25 June 1998, and is classified as a Grade I listed building "of exceptional interest" for its architecture and history. Off-site storage is provided at a second site near Boston Spa in Yorkshire.

The British Library was created on 1 July 1973 as a result of the British Library Act 1972. Prior to this, the national library was part of the British Museum, which provided the bulk of the holdings of the new library, alongside smaller organisations which were folded in (such as the National Central Library, the National Lending Library for Science and Technology and the British National Bibliography). In 1974 functions previously exercised by the Office for Scientific and Technical Information were taken over; in 1982 the India Office Library and Records and the HMSO Binderies became British Library responsibilities. In 1983, the Library absorbed the National Sound Archive, which holds many sound and video recordings, with over a million discs and thousands of tapes.

The core of the Library's historical collections is based on a series of donations and acquisitions from the 18th century. These are known as the "foundation collections", and they include the books and manuscripts of Sir Hans Sloane (d. 1753), whose decision to donate his library and natural history collection to the nation led to the formation of the British Museum. The trustees of the museum also purchased the Harleian Library collection of Robert Harley (d. 1721) for £10,000; Also provided to the library was the Cotton library, the former collection of Sir Robert Cotton (d. 1631); this was already in public possession and had been housed at Ashburnham House, Westminster. These three collections were later joined by the Old Royal Library, donated by George II, and the King's Library of George III.

For many years its collections were dispersed in various buildings around central London, in places such as Bloomsbury (within the British Museum), Chancery Lane, Bayswater, and Holborn, with an interlibrary lending centre at Boston Spa, 2.5 miles (4 km) east of Wetherby in West Yorkshire (situated on Thorp Arch Trading Estate), and the newspaper library at Colindale, north-west London.

Initial plans for the British Library required demolition of an integral part of Bloomsbury – a seven-acre swathe of streets immediately in front of the Museum, so that the Library could be situated directly opposite. After a long and hard-fought campaign led by Dr George Wagner, this decision was overturned and the library was instead constructed by John Laing plc on a site at Euston Road next to St Pancras railway station.

Following the closure of the Round Reading Room on 25 October 1997 the library stock began to be moved into the St Pancras building. Before the end of that year the first of eleven new reading rooms had opened and the moving of stock was continuing. From 1997 to 2009 the main collection was housed in this single new building and the collection of British and overseas newspapers was housed at Colindale. In July 2008 the Library announced that it would be moving low-use items to a new storage facility in Boston Spa in Yorkshire and that it planned to close the newspaper library at Colindale, ahead of a later move to a similar facility on the same site. From January 2009 to April 2012 over 200 km of material was moved to the Additional Storage Building and is now delivered to British Library Reading Rooms in London on request by a daily shuttle service. Construction work on the Additional Storage Building was completed in 2013 and the newspaper library at Colindale closed on 8 November 2013. The collection has now been split between the St Pancras and Boston Spa sites. The Library previously had a book storage depot in Woolwich, south-east London, which is no longer in use.