The collection comprises an extensive family and estate archive, which has been built up through a series of accruals.
Title deeds from the 12th to the 20th centuries relating particularly to the main family estates in Nottinghamshire, Warwickshire and Yorkshire, but also relating to many other counties. There are a particularly significant number of medieval deeds.
Family documents, including wills, marriage settlements, and other documents having an impact on the ownership and succession of Willoughby lands from the 12th century to the 20th century.
Other family papers, including correspondence from the 12th to 18th centuries, 17th-century natural history collections of Francis Willughby and his friend the scientist John Ray, and a volume of family history compiled by Cassandra Willoughby, Duchess of Chandos (1670-1735), featuring transcripts and extracts from papers which have not survived, c.1150-1959.
Personal papers and correspondence of family members, particularly Admiral Sir Nesbit Josiah Willoughby (1777-1849), and Captain Francis D. Willoughby (1819-1846). Disappointingly little family correspondence and personal papers survive from earlier periods.
Estate papers, including accounts, estate correspondence, inventories, maps and plans, surveys and rentals. Given the size and scope of the estates, the maps and plans are relatively few in number.
Legal papers, including the working papers of the family solicitors Messrs. Purchase, Pollock and Treadwell, comprising correspondence, drafts and copies of accounts. Later papers include correspondence, settlement and estate papers relating to the succession of the estate in 1926-1928 and the establishment of the Birdsall Estates Company, 1628-20th century.
Other significant series in the Middleton archive include correspondence, manorial records and official papers. There is a substantial amount of material relating to the family's coal mining activities from the late middle ages and other business ventures.
Further details about the contents of each of the separate accruals can be found in their individual catalogue entries.