The collection is 7660 manuscripts, including 2200 illuminated manuscripts,  more than 14,000 original legal documents; and more than 500 rolls. It was formed by Robert Harley (1661–1724) and his son Edward (1689-1741). In 1753, it was purchased for £10,000 by the British government. Together with the collections of Sir Robert Cotton (the Cotton library) and Hans Sloane (the Sloane library) it formed the basis of the British Museum's collection of manuscripts, which moved to the new British Library in 1973.
The collection contains illuminated manuscripts spanning the early Middle Ages to the Renaissance. There are important early British manuscripts, many from Western Europe, and several Byzantine manuscripts in Greek and other languages.
Among the most significant manuscripts are:
- Lacnunga (Harley 585)
- Harley Psalter (Harley 603)
- Kildare Poems (Harley 913)
- Sumer Is Icumen In, in Harley 978
- Inventory of Henry VIII of England (Harley 1419)
- Gospels of Mael Brigte (Harley 1602)
- Harley 1775, an illuminated Gospel Book produced in Italy during the last quarter of the 6th century
- Minuscule 113 (Harley 1810) - and many others, see the category at bottom of the page
- Harley Lyrics (Harley 2253)
- Harley Golden Gospels (Harley 2788)
- Ramsey Psalter (Harley 2904)
- Book of Nunnaminster (Harley 2965)
- Minuscule 3686
- Harleian Genealogies (Harley 3859)
- Minuscule 104 (Harley 5537)
- Minuscule 505 (Harley 5538)
- Uncial 0121a (Harley 5613)
- Ptolemy's Geography (Harley 7182 & 7195)
- Harley MS. 7334
- Harleian prayerbook (Harley 7653)
- British Library Journal vol. 15 (1989) is devoted to Robert Harley and his collections.
- C. E. and C. R. Wright, eds. The Diary of Humfrey Wanley 1715–1726, 2 vols (London, 1966).
- The foundation collections at the Catalogue of Illuminated manuscripts
- List of medicine and alchemist manuscripts PDF, 192 KB
- Trilingual Psalter
- Harley Bestiary (Harley 4751)
- The Book of Nunnaminster (Harley 2965)