Of the 566 library authorities functioning in Britain and Ireland by 1918, most seem to have published annual reports on their progress, at least at some point following their establishment. So far as they survive and can be traced, these documents, though varying greatly in length, commonly contain a narrative or textual commentary on the progress of the institutions during the report period, statistical and tabular details about the institutions such as information regarding stock, issues, and borrowers, and lists of data such as committee members, donations received and periodicals taken. Some published annual reports documenting the working of public libraries also contain data as to the progress of allied cultural institutions such as museums, art galleries, technical and adult education facilities, and parks and recreation grounds. They were often signed by the local public library committee chairman and addressed to a local council, although some were signed by a librarian or curator and addressed to the library or other committee. In both cases they were accessible to members of the public, and it seems fair to assume that the staff of the institution concerned would commonly have had a hand in their writing. Following the acquisition by the Thomas Parry Library at the University of Wales Aberystwyth of a large collection of historic public library annual reports which the Library Association was on the verge of discarding, the AHRB (Arts and Humanities Research Board) agreed to fund a fifteen-month project between April 2000 and July 2001 to examine the utility of these documents to historians of the period ca. 1850–1919. In view of the lack of thematic historical work on public library annual reports (and indeed on annual reports as a wider genre), this two-part essay will address the genesis and character of these documents from a broad perspective, and assess their impact and significance for contemporaries and for historians in a range of disciplines. After describing the evolution of the public library annual report as a genre, this first paper will discuss the nature of their use to date, and will argue that current common historiographical assumptions as to the limited utility of these documents are flawed. The second paper will assess the value and contemporary importance of the documents in the expansion of the public library movement.