When copyright protection expires the work enters the public domain meaning anyone can freely use it without seeking permission or paying a fee. Since U.S. and international copyright laws defining the time period of copyright protection have been revised several times it often is difficult to accurately determine when a specific item has entered the public domain.
Copyright Term and the Public Domain in the United State is a helpful chart in determining if a work is in the public domain. Created by Peter Hirtle, Cornell University Libraries.
Is It Protected by Copyright? is a helpful tool for determining copyright status. Created by Michael Brewer, ALA Office of Information Technology Policy.
Copyright Renewal Database (Books only): a searchable database of the copyright renewal records received by the U.S. Copyright Office from 1950 to 1992 and for books published in the U.S. from 1923 to 1963.
Copyright Research, Stanford Universities Libraries
Researching the Copyright Status of a Book: Protected or Public Domain? Kenneth D. Crews, Columbia University
Copyright Catalog (1978 to present), U.S. Copyright Office Public Catalog