The library's large collection of periodicals is due in large measure to the work of the Universalist Historical Society to collect any issue it could find, to the historical connection of Harvard Divinity School with the American Unitarian movement, and to the library's continuing effort to collect and preserve Unitarian Universalist publications. Noteworthy examples of the collection include James Freeman Clarke's sometimes annotated copy of The Dial, a complete set of the The Western Messenger (1835-41), a copy of The Theological Repository, edited by Joseph Priestley (1769-88), The Universalist Magazine, edited by Hosea Ballou and others (1819-28), and microfilm of the Auburn University holdings of The Universalist Herald, which augments the holdings of the library. Bound issues of the Leader (1897-1961) and Register (from 1898 to 1961) are available in the periodical stacks, and, together with their previous titles, on microfilm. There are microfilm copies of 150 pre-1900 Universalist titles and over 90 pre-1900 Unitarian titles available in the library or through interlibrary loan. There is also a growing number of older titles that have been digitized.
For insight into the behind-the-scenes activity of the printing of the Leader and the Register are the archival collections of the Universalist Publishing House (bMS 369) and smaller collections of material on the Register (bMS 475, bMS 11081).
The library has an established priority not only to collect but also to preserve these titles, many of which are on brittle, if not crumbling, paper. Four grants from the United States Office (now Department) of Education, beginning in 1978, made it possible for the library to microfilm over 100 periodical titles. An effort to microfilm as many of the periodicals as possible continues for both older and more current titles, including regional and organizational publications. The library now stores the brittle paper copies at the Harvard Depository under ideal temperature and humidity control. The microfilm copies also make it possible for this material to be disseminated to other libraries and scholars through interlibrary loan.