This collection contains many thousands of individual fanzines and pieces of fanzines, amassed by Horvat over the decades. The fanzines in the collection deal mostly with science fiction and related subjects, although there are zines on other genre topics, such as mysteries. The collection chronicles the evolution, growth and development of science fiction fandom from its early organization in the 1920s through the various iterations of fandom that occurred over the subsequent decades. Together the materials in the Horvat collection document the history of SF fandom over the course of its first century of life.

In physical terms, the number of individual items represented here is on the order of 15,000-20000 pieces.

Information is generally entered: Title (Place of Publication) editor(s). Issue number (date of publication).

ALS = autograph letter signed; TLS = typed letter signed.

Bibliographies, lists, booksellers catalogs, and similar materials have been separated and are listed in Series IV, Fanzine Bibliography.