Many university libraries provide subject guides that give information on grey literature and suggestions for databases. ROAR and OpenDOAR are directories of open access institutional repositories and subject repositories, many of which contain some grey literature. Several academic search engines exist to combine the open data provided by such open archives through OAI-PMH, as well as records from publishers deposited in CrossRef and other sources. They include BASE, CORE and Unpaywall, which indexes over 20 million open access publications as of 2020.
